pH, KH, CO2 relationship - please confirm my 'conclusions' - The Planted Tank Forum
 
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post #1 of 9 (permalink) Old 02-25-2004, 04:04 PM Thread Starter
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I'm about a week away from finally setting up my first 'real' planted tank....equipt with an automated, pressurized system. Because my tap water has a GH and KH of about 15 deg (dolomite aquifer), I use an RO unit. For my non-planted tanks, I've been using 2/3 RO and 1/3 tap to achieve 5 KH and 5 GH. I have Kent RO right and baking soda, but can't see a reason why mixing [treated] tap water wouldn't achieve the same results as adding those two supplements:?. Thoughts?

Anyway, back to the subject... Given that I use the same 5 KH water for my planted tank - setting the pH controller to 6.8, are the following conclusions correct. *Of course I would never DO either of these things. - I just want to confirm that I understand the mechanism.

1. I do a 50% water change with strait tap water. KH will double (to 10 KH) and my CO2 system will poison my fish [plants are fine].

2. I do a 50% water change with RO. KH will be cut in half (to 2.5 KH) and my plants will not receive an optimal amount of CO2 [fish are fine].

Given these two extreme scenarios, would you agree that if error must exist, it would be better to error on the side of more RO (ie - Lower KH)?

Ted
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post #2 of 9 (permalink) Old 02-25-2004, 04:24 PM
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I've been told tonot use alot of R/O becuase it lacks the nutrients that plants need. but my water is harder than yours. KH was about 20, and GH was about 25.

When I filled my tank, I used 10 gal RO and 20 tap (it's a 29 gallon).
When I do water changes, depending on how much water I change, I use 1/2 RO 1/2 Tap.
My paramaters stay about the level of:
kh=11deg
gh=18deg

My fish and plants seems to be doing just fine like this.

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post #3 of 9 (permalink) Old 02-25-2004, 05:37 PM
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Ted, here is my interpretation of what would happen:

1. You do a 50% water change with tap water, and KH doubles to 10 dH. The added alkalinity and the addition of low CO2 tap water initially drives your pH up. The additional carbonate buffering capacity would make it harder to drive down your pH back down...you would have to add a lot more CO2 than at a KH of 5 dH. Assuming your CO2 diffuser is adequate and you don't start outgassing CO2 through the water surface as the the concentration in the tank climbs, you will eventually reach the controller pH of 6.8. The dissolved CO2 in the tank will be high (I don't have my chart handy for the exact amount) and would/could stress or kill your fish.

2. You do a 50% change with RO water, and KH drops to 2.5 dH. Initially, the drop in KH should cause your pH to also drop, but you are adding low CO2 RO water, which should cause the pH to rise. Will the two processes balance each other?...Don't know; it's hard to say. Your carbonate (and total) buffering capacity is now half of what it was, so CO2 injection will alter your pH relatively quickly. Depending on how far and how fast the pH changes, this could stress your fish and lead to disease or kill them outright. Dissolved CO2 in the tank will be sub-optimal for plants so they will suffer.

Given these two scenarios, I would add carbonate and total hardness to the RO water to get the desired value of 5 deg. KH and 5 deg. GH **before** I added the water to the tank. That way there is no "error."

BTW, why don't you set up these two scenarios as "experiments" in a bucket? I would be very interested in your findings!!!

29 gal planted tank, 30" FW Aqualights w/ 1x 65W 6700K CF & 1x 55W 9325K CF, 20-lb CO2 w/ Milwaukee SMS122 controller, reverse-flow UGF (bah!), gravel substrate (also bah!), 4 Praecox, 3 Ottocats, 1 SAE, 1 Bristlenose cat, 2-3 C. japonica, 5 Red Cherry Shrimp, & too many detestable pond snails!!!
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post #4 of 9 (permalink) Old 02-26-2004, 11:13 AM
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Why not just do a water change with 25% of each (RO and tap) water and avoid the headache?

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post #5 of 9 (permalink) Old 02-26-2004, 02:49 PM Thread Starter
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hehe.

Thanks for the replies guys. Perhaps I should have left out that whole "error" part. In trying to be thorough, I think I may have strayed off the topic a little too far ops: :lol: .

Splash addressed my main concern.....what will happen (not what to do to avoid letting it happen). Like I said, I just want to confirm that I understand what is going on. For part 1, you've confirmed what I had thought. But for part 2, you have suggested that the (pH KH CO2) mechanism might not be as simplified as I had thought......very interesting. I'll do some more reading, and perhaps even run that experiment you suggested.

I'm not looking for problems, I assure you . I'm just the kind of nut who likes to understand things inside and out.

Thanks again.
Ted
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post #6 of 9 (permalink) Old 02-26-2004, 03:24 PM
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What you wanna do is mix your tap and R/O water in a large container...like a 35 gallon plastic trash can, heavy duty, purchased new JUST for this purpose. Then you can make sure pH, Kh and Gh are all good. Then use a small, inexpensive submersible pump to actually fill your tank.

This way you can also use a 150 watt heater to bring the water temp to the right level too, so there's no pH, Kh, or temp shock to the fish.

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post #7 of 9 (permalink) Old 02-26-2004, 04:07 PM
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I'm not sure about splash's answer on #2. If you are using a ph-meter controlled co2 system then your assumption is correct. Your system will shut off at the same pH you had it set for previously, thereby injecting less co2 into your tank because of the drop in KH.

George


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post #8 of 9 (permalink) Old 02-26-2004, 05:07 PM
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i agree with splash's and Georges interpretations of what would happen.

As an aside, there is no difference in using tap water or RO right and bicarb if all you are worried about is hardness. However the tap method will also reintroduce all the nasties that the RO unit has taken out, such as chlorine, heavy metals and pesticides etc. these aint good for your fish.

the easiest way to make sure you dont balls things up is to get an electronic TDS meter. This will instantly give you a reading for the total dissolved solids, so you can tell if you have accidentaly used pure RO or pure tap water. RO should be very low (<20ppm TDS) and it sounds like your tap water will be way up in the 4-500ppm range. Therefore if you have done a mix, it should be around 150-200ppm. makes for a nice failsafe if the last thing you do each water change is to check TDS.

HTH

KB :-)


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post #9 of 9 (permalink) Old 02-26-2004, 08:00 PM
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Actually, George, I think I said that too, although my wording might not have been as explicit as yours. But CO2 injection will take some time to re-establish equilibium after pouring in the RO water, so I was first considering what would happen immediately after the RO water was added.


Quote:
Originally Posted by George Willms
I'm not sure about splash's answer on #2. If you are using a ph-meter controlled co2 system then your assumption is correct. Your system will shut off at the same pH you had it set for previously, thereby injecting less co2 into your tank because of the drop in KH.

29 gal planted tank, 30" FW Aqualights w/ 1x 65W 6700K CF & 1x 55W 9325K CF, 20-lb CO2 w/ Milwaukee SMS122 controller, reverse-flow UGF (bah!), gravel substrate (also bah!), 4 Praecox, 3 Ottocats, 1 SAE, 1 Bristlenose cat, 2-3 C. japonica, 5 Red Cherry Shrimp, & too many detestable pond snails!!!
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