Flaws in Kh chart regarding CO2 content - The Planted Tank Forum
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post #1 of 30 (permalink) Old 09-17-2014, 08:22 AM Thread Starter
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Flaws in Kh chart regarding CO2 content

So in an attempt to help keep my water more stable doing my water changes to help get rid of BBA algae in my pressurized set ups, I decided to try an estimate the amount of CO2 in my R/O water using various samples in an attempt to keep my CO2 more stable during water changes even at night . In 1 sample I simply tested pure r/o water with regency test kit. It registered nearly 6.0 which I figured inaccurate since there was no buffer in the water.
So I then added 2 kh to the sample using sodium bicarbonate. Now I registered 6.6. This should mean I have about 15 ppms of CO2 in freshly made r/o water. So you would think I should hold that level of CO2 if I double the kh , right? So 4kh yielded a ph of 7.3 or close to it. That gives me a new CO2 level of only 6 ppm's???? I then repeated both kh value samples this time with a small amount of CaSO4, & MgSO4. The results were very similar. I figured the extra ions in the water with the buffer would balance the water better an give a more accurate reading. But it did not. What this tells me, correct me if I am wrong, is that the kh CO2 estimator chart is totally useless! Unless there is something I am missing about what I did that someone could explain better to me?
Let me give some examples why I also feel the chart is meaningless. I have another tank that has dropped to near 6.2 with a kh of 5! That tank has no ph altering substrate nor does it have driftwood! That should raise my ppms of CO2 to 94 PPMs. I know its an estimate but 94ppms should have gassed the fish! But they still are living on an doing fine.
Is there a better way I can measure the r/o water CO2 content before or even after mineralization which I always do anyway before w/c's? The reason I ask this is because if CO2 needs to be stable than even in ambient light with large CO2 swings should induce BBA to grow if they are strictly stimulated by CO2 fluctuations which I do believe but I also believe there are other factors involved as well like high organics.
Thanks

Last edited by Hardstuff; 09-17-2014 at 08:15 PM. Reason: typo
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post #2 of 30 (permalink) Old 09-17-2014, 08:41 AM Thread Starter
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I guess mainly what I am asking is: I would like to know do to my lack of chemistry is : What happened to 6 or 7 ppms of CO2 when I added the extra sodium bicarbonate to the sample? Does the CO2 disassociate from the sample after adding the buffer?
Should R/O be degassed before morning lights on w/c? Or left alone to keep more gas in it? These are questions I have been pondering for awhile. I know that my tap water an r/o water degassed experiments generated a BBA bloom recently indicating an influence from CO2 swings. So I usually have better results from non degassing r/o remineralized water than from degassed but I am still working on that. Its a work in progress for me but I would like to get some fed back from folks using R/O water an when or what they do to prep their water if nothing else. I would also like to get feed back from folks using regular tap water as well an the results in inducing BBA or not from what they experience from doing water changes to help reduce or eliminate BBA by keeping CO2 stable?
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post #3 of 30 (permalink) Old 09-17-2014, 08:46 AM
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We Gots KH's and pH's, but dowe got CO2?

I think the fallacy you are running into is that changing the the values of the carbonate, just as adding acid or baking soda does not change the amount of CO2.

I am too tired now, if no one else has come along I will do my best to offer an explanation you will find reasonable and can test for yourself.

As a by the way (BTW in online speak) RO or pure water should be about pH 7 at 25C (77F), temperature being the only variable.

Joe
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post #4 of 30 (permalink) Old 09-17-2014, 08:57 AM Thread Starter
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I do realize that adding a base or acid raises an lowers ph. I am not talking about the relationship between acids an bases but CO2 content in water verse kh an ph levels regarding the so called CO2 level chart that uses kh values an ph to determine CO2 in the water. If I have a sample that is supposed to have 15ppms of co2 in it than what happens to the CO2 when more sodium bicarbonate is added? I am sure its a chemistry thing. BUt to me it does not make sense that adding NaCO3 will make the CO2 outgas but change it to something else. Can someone explain what happened to the CO2?

Bump: OK, some of my old chemistry is coming back. The sodium bicarbonate neutralizes the acid in the CO2 . So what is happening the current CO2 is getting depleted from the sodium bicarbonate reaction. So to answer my own question, Degassing should not be necessary since adding buffer will deplete the CO2 even more when mixing.
Knowing this what time would be the best time to avoid CO2 swings? In theory NO time! Lights out morning or night but if the ambient light is too high than the water change could induce BBA?
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post #5 of 30 (permalink) Old 09-17-2014, 03:31 PM
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Freshly made RO? Allow the water to sit for a good 24 hours, in a shallow wide pan etc. Then measure the TDS, it should be 10 or less uS.

How you measure pH needs addressed also. Very pure water needs a different pH measurement probe. Not the typical pH probes folks often use.

If the water is less than 10uS, then add the bicarbonate to get 1 KH, 17.86 ppm.

Wait a day or so to equilibrate and dissolve. Now measure the pH.

You can add say 4 KH to another sample and do the same thing, the CO2 content will be the same. Only 1/400th of the KH is going to be converted to carbonic acid, the rest is going to remain as CO2[aq].

So we ignore the carbonic acid part and simply assume the CO2 gas is all dissolved CO2[aq]. You cannot remove and soften the KH using CO2 gas, some make this error and think adding CO2 removes KH by magic or something. Would be nice if that's all we had to do to remove KH, but everyone that uses CO2 gas know that' is just not the case.

So start your KH up a bit higher. The CO2 table is correct, but it can error on trhe lower side than actually real values, but NEVER on the higher side of what is actually contained in the water. Unless you have a faulty pH meter etc. And most of those errors will push the pH down making you think you have more CO2 than you actually do.

Some folks simply use relative pH drop and then eyeball from there. I think this is the best approach given all the other issues and trade offs.
With RO, as long as the RO is maintained, you can do well with the pH/KH/CO2 chart. Add about 1 degree of KH, you do not need to add more for plants or other reasons.

Fish perhaps of some species etc.

Or if you blend the tap with the RO to get say 3-4 KH.




Regards,
Tom Barr
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post #6 of 30 (permalink) Old 09-17-2014, 07:14 PM Thread Starter
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Thanks Tom. I have always used a kh of 3 or 4 with pretty good results an stable ph. My r/o is very good. My TDS on a good day is 16 since my tap has very high TDS usually around 530. So I am using a membrane thats is supposed to be 99% rejection but since the water is very warm out of the tap its not realistic to expect 99%. That being said 16 is good for what I am working with.
I am still confused on how the CO2 is still present in the same concentration because at 6.3 or close to it with the kh of 1 I should have about 15ppms of CO2 going by the chart. Now when I raised the kh to 4 degrees I should only have about 6 ppms of CO2 as my ph reads 7.3-7.4. Now I am only using regency ph test kit. I only had a digital for a loan for awhile . I found the digital to not hold that well. The regency seemed more consistent. I am sure there are better units out there but I do not want to get into that. I use to work in a lab an I used expensive ph meters to measure deionized water. I just want to know why the co2 content changes when adding kh according to the chart?
You say the CO2 stays the same. But if you compare the levels of co2 to the new kh level they are not the same. What is the difference in taking a ph reading in a planted pressurized tank an going by the kh chart to figure out what your CO2 content is? I know CO2 is an acid so adding kh would reduce this acid being the co2. I guess its the aqueous part I am not getting? After a day or so would it be more accurate? But then you will lose a lot of gas?
You say wait 24 hours to stabilize . Finally thats the answer I have been looking for this as well. There is hardly any info on stabilizing r/o water on the internet. Very little on how to add natural trace back in as well. Thats another subject.
Finally, What would the best time to do water changes using this method to avoid a BBA bloom because of CO2 flux?????? Would using freshly made r/o be better since it has more CO2 in it going into a pressurized set up before or after lights on? Or would you want to age the water an degas it an change at night only or in the morning before lights on????? Either way, there will be some CO2 flux. If enough ambient light is present you would figure a bloom is eminent? BBA seems sensitive to even slight CO2 changes.
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post #7 of 30 (permalink) Old 09-17-2014, 07:54 PM Thread Starter
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Not to hijack my own thread, but when I use to live on Long Island NY, I never grew BBA algae at all. I knew nothing about growing plants other than they needed light CO2 an a decent CEC exchange substrate. I actually limited my PO4 an NO3 an had medium consistent light with a pathetic diffuser that caused huge swings in CO2 an ph all day long! Yet I grew NO BBA! I grew hair algae at times , GDA, GSA, BGA but zero BBA! I had some good growing times an some bad. But the bad usually was during periods of neglect. Like not doing water changes. When the tank was doing well ( 45 gallons) , I usually cleaned the filter every month like clock work. The water was very soft, 10,000 year old or so glacier water from wells, probably low amounts of organics.
Since moving to az I have been battling BBA all the time. Whats the difference. Surface water containing high amounts of organics. Yet back in New York I could do what ever I wanted to. Large water changes during lights on , it did not matter, BBA would not grow. So to say its just CO2 based alone, I do not agree. But then again, 99% of the water folks use was not the water I used back in NY. So we are back to just CO2 again causing swings which I agree unless you are lucky enough to have water low enough in certain trigger fuel source organics. Of course just my opinion. But Why did BBA not grow in the 45 which had huge swings in CO2 even after 3 years?
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post #8 of 30 (permalink) Old 09-17-2014, 08:20 PM
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Smile Oh My

Since the expert has spoken and I admit I am having a difficult time following your reasoning, let me just say that the table (KH, pH, CO2) and the equation from which it is derived are indeed reasonably and demonstrably accurate for systems where the KH (alkalinity) is carbonate.

If you know any two quantities the table/equation provides the third. In a system at equilibrium.

I (initially anyway) thought you were trying change the amount of CO2, or thought it should change when altering the KH and therefore the pH.

My mistake.

Joe
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post #9 of 30 (permalink) Old 09-17-2014, 08:54 PM Thread Starter
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Thanks Joe, you were right an wrong. I was just trying to figure out what the freshly made r/o water CO2 content is from my filter in hopes of not causing CO2 swings when doing water changes. The problem is, without any kh all that goes out the window. Even on the chart . If you plug in a kh of zero you get a reading of zero from the CO2 side regardless of ph. So I then figured out there has to be a reason to have some kh to estimate CO2 in water. Thats when I added 1 or 2 kh to start to get a starting point in CO2 content. But to me, what gets confusing the most is that Tom says the CO2 stays the same regarless of adding kh yet the table says it changes. Now I am afraid of doing water changes at all because I do not know what CO2 level I have going into the system?
24 hours later the ph regency still reads about 7.4 which it read last night. Nothing has changed. So what would be my CO2 level now in my r/o buckets? According to the kh charts it should be about 6ppms or less with a kh of 4. So you would think that would induce BBA if my current CO2 level in the tank is around 45ppms of CO2 gas?
So according to Tom, if you add kh to a given sample the CO2 stays the same. Is it the aqueous part of solution I am not getting, an if so how do you calculate that? But clearly according to the kh charts the CO2 goes down when adding kh . Unless using a CO2 meter it seems in this case it is impossible to calculate CO2 levels after adding kh?
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post #10 of 30 (permalink) Old 09-17-2014, 10:19 PM
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Smile Water Changes Stablize the RO

Tom is correct, there is no change in the amount of CO2.

KH, carbonate hardness is simply the buffer that resists the change in pH, one of the reasons for water changes is to keep that buffer. Another term for KH is temporary hardness, because, unlike general hardness the KH can be used up.

Small amounts of KH are dangerous. Just read the posts here and see how often someone has a disaster in a tank that has been operating just fine for years and of course it turns out they thought they didnít need water changes or other basic maintenance.

Water changes, reconstitute your RO (aerate, add stuff to get the GH, KH and pH right).

Joe
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post #11 of 30 (permalink) Old 09-17-2014, 10:42 PM Thread Starter
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I do all that stuff already. I add gh booster an Kh buffer. I add extra trace for the fish mostly. I aerate the r/o from time to time but still not sure if that is necessary since it takes the co2 away causing bigger swings during water changes. I have aerated before with straight tap water an got my biggest bba bloom to date. So thats why I was thinking of going back to fresh r/o remineralized an right into the tank at night. That way the CO2 in the system should stay more stable even though the lights are out. But I am not sure about that. I do feel the ambient light if bright enough during water changes is a problem even if aquarium lights are off.
I do believe the kh ph charts are correct. I just am not understanding what happens to the CO2 when adding the kh on the spot regarding ph regency tests to try an figure out the CO2 content. I realize it is what it is regarding a sample out of the tank. But if you suddenly raise your kh you will not know what your CO2 level is at that moment until the tank settles in the next day. Correct me if I am wrong please.
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post #12 of 30 (permalink) Old 09-18-2014, 02:46 AM
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Smile Am I close?

I am not sure I am getting this. Are you worried about the CO2 swings during water change? If that is the case do not, the CO2 will rebound quickly enough. As it is there should be some just based on plant usage. The important part is system stability in this case buffering. Give the tank time to come to equilibrium.

Letís try it this way. Take the issues separately, prepare the RO that includes aerating, I like a shot of lemon juice, some folk buy electrolytes, then add whatever amount of stuff you need to match the general hardness.

Then add baking soda to give the KH, carbonate hardness you desire. Unless there are some particular fish or plants with special needs I recommend at least dKH 4, about 70-ppm Calcium carbonate (CaCO3), remember we are measuring KH in equivalent units.

You are actually adding 5-grams, a teaspoon of baking soda for every 10-gallons (about 132-ppm NaHCO3). Though obviously we are interested in the HCO3- constituent so you have actually added about 30-ppm Na+ and about 102-ppm HCO3-, the water is strong (polar) enough to break the ionic bond. So for our buffering purposes 102-ppm HCO3- is the equivalent 71.6-ppm KH, which is 4dKH.

I actually recommend 6-dKH, 7.5-grams baking soda for each 10-gallons of RO. Let the system come to equilibrium, it may take a while. The water will settle around pH 7.8 or so, which in accordance with the chart will give you a CO2 value of 3-ppm or so. That is what the atmosphere provides.


Joe

Bump: Also do not try to take on everything at once if we are together this far that is okay.

Now once your reconstituted water comes to equilibrium if you used 4-dKH for the buffering the atmosphere should provide you about 3-ppm CO2.

So free of charge you are getting around 3-mg of CO2 in every liter of water. Looking at the chart I think you should find the pH will come in around 7.8.
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post #13 of 30 (permalink) Old 09-18-2014, 04:33 AM
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Originally Posted by Hardstuff View Post
Thanks Tom. I have always used a kh of 3 or 4 with pretty good results an stable ph. My r/o is very good. My TDS on a good day is 16 since my tap has very high TDS usually around 530. So I am using a membrane thats is supposed to be 99% rejection but since the water is very warm out of the tap its not realistic to expect 99%. That being said 16 is good for what I am working with.
Just change the membrane and prefilter and the Carbon prefilter(the most). Back flush every month or two also.

Keep an eye on the TDS, it's easy to check.

There's no need to add that much to the KH, only if you blend, then 3-4 KH is a good target.

If you use 100% RO, then target 1 KH with the baking soda etc.
No need to add 3-4 degrees worth.

Quote:
I am still confused on how the CO2 is still present in the same concentration because at 6.3 or close to it with the kh of 1 I should have about 15ppms of CO2 going by the chart. Now when I raised the kh to 4 degrees I should only have about 6 ppms of CO2 as my ph reads 7.3-7.4.
There's no errors in the chart(my assumption), there are errors in your measurement and assumptions of the measurements.

If the tank water is truly fully degassed, then the CO2 would be the in the 2-3 ppm range or thereabouts. My tanks had 20 ppm after 14 hours of no CO2 added even with high flow rates. I switched to sump/wet/dry filters, now the water degases to 2-3 ppm after 45-60 minutes after the CO2 is stopped.

Canister filters stink for degassing CO2 and adding O2.
This was true on 6 different tanks. Not just one.

O2 levels shot up also.

You know something is not right, but you doubt what it is..........
So let me pose a question to you: do you think the chart is wrong, or something that you did is wrong?

Which would be a better assumption? I'll let you answer that on your own.

Quote:
Now I am only using regency ph test kit. I only had a digital for a loan for awhile . I found the digital to not hold that well. The regency seemed more consistent. I am sure there are better units out there but I do not want to get into that. I use to work in a lab an I used expensive ph meters to measure deionized water. I just want to know why the co2 content changes when adding kh according to the chart?
See above. Your answer in the paragraph right here.

Standard American Marine type pH meters run about 90$ and replace the probes about once a year. These tend to be very consistent.

Stray electrical current, non shielded ballast and other sources will reduce the pH on pH probes, so turn everything off and THEN, take your reading. If it's drops when you turn things back on, then your know there's stray current impacting the pH reading.

As you can see, and as I've told folks 10,000 x in the past, CO2 is not a simple thing.

"My plants have algae or they are stunted or they do not grow for X, Y and Z but my CO2 has to be correct"

If I had a nickel.............

Then they add more and still do not get the results they want, or gas their fish etc.

But measuring it and adding enough O2, degassing rates, good circulation etc all play a large role there also.

Most only hear, "add more gas".

I'm saying this for the benefit of others that might read this as well as you.
Some of these things might seem obvious to you, but no to others.

Still, I get dogged by simple stuff all the time with CO2. So I try and be careful if I see anything not quite right and always question myself and my own mistakes/assumptions rather than the chart.

Quote:
You say the CO2 stays the same. But if you compare the levels of co2 to the new kh level they are not the same.
Well, did you add more CO2 or take any out?
If not, then it's somewhat safe to assume.............they should be identical.
How you measured it? Perhaps not a safe assumption.

Quote:
What is the difference in taking a ph reading in a planted pressurized tank an going by the kh chart to figure out what your CO2 content is? I know CO2 is an acid so adding kh would reduce this acid being the co2.
No.

You did not understand the last post.

CO2 gas forms CO2[aq] in water. Only as tiny fraction of said gas will combine with the carbonate to form carbonic acid. 1 out 400 molecules of CO2 will form the acid with the KH.

You seem to assume it all does this. If that was the case, you'd have no KH left. Think about it. Measure your KH with and without CO2 gas addition. It'll be the same. Adding CO2 gas enrichment to an aquarium is very different than natural systems. We want CO2 [aq], we do not care about pH directly or KH really.

Quote:
I guess its the aqueous part I am not getting? After a day or so would it be more accurate? But then you will lose a lot of gas?
Yes, the [aq] part messes up folks often, this is one of the harder things for hobbyists to understand, so do not feel bad. You want the water to equilibrate with the air, that acts as a reference point.

Fully degassed.

Now if you add say enough CO2 gas to knock the pH down 1.0 units from THAT fully degassed reference point, now you likely have pretty close to 30 ppm added extra.

You can check this with thr chart at 2-3 ppm and then look down 1.0 pH units and see now you have about 30 ppm in most KH's. The KH levels/degrees do not matter. A KH of say 10 degrees has a pH/KH of

10KH: pH 8.0 = 3 ppm CO2
10KH: pH 7.0 = 30 ppm
pH of 6.8 = 47.5 ppm of CO2 etc.

A KH of 1: pH 7 = 3.0 ppm
KH of 1; pH of 6.0 = 30 ppm.

In BOTH cases we add the same volume of CO2 to drop the pH because we adding mostly 99.75% CO2[aq].

We are adding CO2 for the plants, not to monkey with KH, acid base equilibra, some Baloney about pH stability etc. Plenty of Hee haw on the web on this topic. Believe me.

Quote:
You say wait 24 hours to stabilize . Finally thats the answer I have been looking for this as well. There is hardly any info on stabilizing r/o water on the internet. Very little on how to add natural trace back in as well. Thats another subject.
Finally, What would the best time to do water changes using this method to avoid a BBA bloom because of CO2 flux?????? Would using freshly made r/o be better since it has more CO2 in it going into a pressurized set up before or after lights on? Or would you want to age the water an degas it an change at night only or in the morning before lights on????? Either way, there will be some CO2 flux. If enough ambient light is present you would figure a bloom is eminent? BBA seems sensitive to even slight CO2 changes.
Well, 24 hours is okay, some use 48 hours, I use a shallow thin pan to do it. This makes degassing rates faster as shown by Fick's 1st law of diffusion.
More surface area and then less depth of the layer requiring to be degassed. Think about it like this, what cools off faster after taking out of an oven: a thin cookie or a thick brownie? Clearly, the cookie.
Same type of thing here.

If you use CO2 gas,m the more times you do water changes, the better as a rule, the time of day is not that important really. I do mine before or right after the lights come on personally.

BBA blooms seem related to CO2 issues and folks not adding enough, but adding some. And there's often a delay.............maybe 1-2 weeks, then it becomes a real PITA to keep up with it, but if you correct the CO2, then the bloom and NEW growth stops. Now you trim off the infested plants and the new growth will remain clean.

For BBA on non live materials, peroxide or scrubbing or removing and dip in hot water, Excel etc, if you can spray larger non live materials with Excel, without hitting the live plants, when you do a large water change, that will work well also.

Once you master CO2 well, BBA is a non issue really, you might screw something up and then need to beat it back, but it's just par for the course and you know why and you fix the CO2 issue and try to avoid them in the future.

Example, sometimes my CO2 gas tanks run out and I do not catch it for a few days. Hard to view the gauges under the stand etc. I can see the plants are not growing so well as before. So I check the CO2. Ferts and light can be added consistently pretty easily.

Happens to everyone. Anyone that say otherwise just needs to be patient and it'll happen to them.




Regards,
Tom Barr
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post #14 of 30 (permalink) Old 09-18-2014, 06:33 AM Thread Starter
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Thanks Tom. I guess its the aqueous part that I am having a hard time understanding. I also probably confused some folks but I really was not pitching trying to change the kh using CO2. Not sure where that came from. I just wanted to know what is going on in solution with the CO2 after adding a buffer, period. I am going to have to open my chem book now because I do not understand the aqueous part. The chart is supposed to tell you the CO2 level if ph & kh values are known. I will review this an thanks for the explanation.
Well one tank right now is killing it. Very nice plant growth stable CO2 an nutrients, I just want to keep it that way, thats where the degassing an water change time came from as well. I guess I will go back to degassing even though I run pressurized as well as low tech tanks. My big tank had BBA problems related to CO2 since than I have corrected the issues but the tank is still cycling through early tank issues but green algae is growing back which is a good sign. Plant growth is picking up as well. But I I do not agree that CO2 is always part of algae control . For instance, using r/o is tricky to balance, all it takes is one nutrient down an growth slows or stops regardless of CO2. After trial an error I figured with the help of others as well that my K was out. The tank an fish responded immediately to the addition of K. Better O2 production from the plants an better growth an the fish were happy again. Green algae is growing fast as apposed to brown or red. Cleaning filters an like you say, concentrate on good plant growth.
Regarding gassing fish or not having a handle on gas. For me, I never gassed a single fish in the 5 years running pressurized tanks, nor Do I ever plan on that. Deficient at times, for sure! But thats getting a lot better as well. I find that Getting a handle on the CO2 is more about the type of equipment being used an knowing your tank an not making mistakes. If you have poor CO2 equipment than you will have issues not including the other variables like light an nutrients, filtration.
Canisters being poor at degassing CO2. If thats what you meant , I find them good at surface agitation an O2 aeration using spray bars. But they may need some help in bigger tanks. Sumps are nice to clean the film from the surface but waste a massive amount of CO2. I know you have devised methods to reduce this but there is still big wastes of gas using these filters but they will add O2 which is good. Trade offs I guess. I would like to have one one day. I do not run tanks big enough for them right now. They are good filters but are better on big tanks. The spray bars do need fiddling an attention to get it right on the canisters.
I noticed Tom you did not comment on the long Island water with NO BBA. Back then my CO2 equipment was poor, an my knowledge of plant nutrition was poor as well compared to now thanks in part to you. But I did not grow any BBA in its 3 year run in that tank an my CO2 was all over the place, so what was going on there????
As I have stated before CO2 seems to trigger BBA but its fuel energy source appears to be leaning towards certain organic compounds. Thats the only difference in my AZ water verse NY soft 10000 year old glacier water down 2000 feet below the surface verse forest fire's down trees, an everything else including dead birds & fish water. Lots of junk floating around in surface water. An yes its what 99.89% of the kind of water that most of us use in our planted tanks besides some really lucky folks.
I do appreciate the advice on water changing Tom an Joe. Thanks I will take your advice on that for sure an rest easier at night, provided I lay off the caffeine. Thanks again Tom keep the good work up you have done for us. I would be interested in what you have to say about the soft deep ancient well water I had with zero BBA?

Last edited by Hardstuff; 09-18-2014 at 09:32 AM. Reason: typo
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post #15 of 30 (permalink) Old 09-18-2014, 07:58 PM
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Originally Posted by JoeRoun View Post
Tom is correct, there is no change in the amount of CO2.

KH, carbonate hardness is simply the buffer that resists the change in pH, one of the reasons for water changes is to keep that buffer.
KH does not act as a buffer against pH changes when CO2 is added. Whatever the KH is, if you add 100 ppm of CO2 to the water the change in pH will be the same. With high KH the actual pH will be higher than when you have low KH. I wish we could give up calling KH a buffer.
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Another term for KH is temporary hardness, because, unlike general hardness the KH can be used up.
When discussing planted tanks, as we are, both GH and KH can be reduced by plants using the ions that make up each form of hardness.
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Small amounts of KH are dangerous.
Any KH from 1 on up will work fine for the plants, but some plants do better with low KH and some do better with higher KH. It is barely possible to achieve a KH low enough, using tap water, to be "too low". The "disasters" attributed to low KH are caused by something else.
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Just read the posts here and see how often someone has a disaster in a tank that has been operating just fine for years and of course it turns out they thought they didn’t need water changes or other basic maintenance.
Water changes are for removing stuff from the tank water, not maintaining stuff in the water.
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Water changes, reconstitute your RO (aerate, add stuff to get the GH, KH and pH right).

Joe
There is no "just right" set of water parameters, because both fish and plants can do well with a range of values of typical parameters.

Sorry to pick on you I am on a crusade against the idea of KH being a buffer against pH changes caused by adding CO2 to the water. Oops! Sorry, I see another windmill to attack!!

Hoppy
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