Stupid question about Co2 and Size of Tank - The Planted Tank Forum
 
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post #1 of 8 (permalink) Old 10-04-2015, 02:49 AM Thread Starter
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Stupid question about Co2 and Size of Tank

Okay then. Back when I was a young buck(haha) and dosing Co2 I could never afford the bigger tanks so never worried too much about the 1-2 bubbles per second thing. That's what I used. But now that I have a bigger tank 105g, I'm looking for some kind of info on how much more I should be using and I can't seem to find much of anything relating to Co2 quantity vs. size of tank. What am I missing?
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post #2 of 8 (permalink) Old 10-04-2015, 02:57 AM
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It's because each tank is different. Just cause my 90 gallon is balanced at 3 bps doesn't mean your 105 will be balanced at 3 bps.

I think I keep reading that you want like 30 ppm co2. Though I'm not 100% sure.

Either way, I think a 20g tank should last 9-12 months, but again I'm not sure, just what I've read.
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post #3 of 8 (permalink) Old 10-04-2015, 02:58 AM
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I would buy a drop checker to ballpark the CO2 level. It is not very expensive. It is slow but it will definitely help you understand the relationship between BPS and the amount of CO2 available. I always use half the amount of liquid in the drop checker to save money. It does the job.


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post #4 of 8 (permalink) Old 10-04-2015, 04:32 AM
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Quote:
Originally Posted by Bainreese View Post
Okay then. Back when I was a young buck(haha) and dosing Co2 I could never afford the bigger tanks so never worried too much about the 1-2 bubbles per second thing. That's what I used. But now that I have a bigger tank 105g, I'm looking for some kind of info on how much more I should be using and I can't seem to find much of anything relating to Co2 quantity vs. size of tank. What am I missing?
What you need is a drop-checker to monitor the CO2 level in your tank.

The bubble-counter gives you an idea of your supply of CO2 into the tank. This supply may or may-not be sufficient for the plants or even be higher than what the plants can consume. So either the plants will not get all the CO2 they can consume, or CO2 may be at an excess and build-up in the tank threatening your fishes, or you may be lucky enough to be supplying the right amount.

How much CO2 your tank can consume depends on the mass of foliage in your tank (this may vary with trimming and growth), and the type of plants you have there, and also the intensity of your lights, and also again by the plant nutrients available. So you see that it is impossible to predict CO2 consumption/requirement with certainty and the size of the tank is only a very minor variant.

A drop-checker is a in-tank gadget which will let you know if the CO2 you are supplying is building up without being consumed and therefore the supply is threatening your fishes. Its telling you to reduce the CO2 supply. By observation and trimming your CO2 supply; you will soon come to know the exact supply rate your tank needs - and the rate that you should want to see in your bubble counter.

The drop-checker is only a ph indicator in a specific alkaline solution with a captive air head which is connected to the tank water. This air-head has the same equilibrium composition as the gasses in the tank water and there the indicator solution also has the same equilibrium of gasses as the tank water.

Now if CO2 level increases in the tank - then the CO2 level will increase in the captive air above the drop-checker indicator - and also the CO2 level in the indicator solution itself. With the increase of CO2 level in the drop-checker solution - the ph of the solution starts to decrease and the colour of the indicator starts to change. The colour of the indicator will give you the CO2 concentration in the tank, in ppm, from the comparison with the colour chart.

So do get yourself a drop-checker and end your uncertainties.

If you have a choice, you have a problem, till you elect your choice. No choice, no problem, only consequences, learn to live with them.

Last edited by essabee; 10-04-2015 at 04:34 AM. Reason: add
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post #5 of 8 (permalink) Old 10-04-2015, 05:09 AM
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Bainreese,

it's NOT a stupid question, but it is also not easy to answer. Solubility is a physical parameter that is independent of the tank size.

When you blow CO2 bubbles into your tank, the transfer rate of CO2 from the gas phase into the water is only dependent on the interchanging surface. That means that the smaller the bubbles the more surface you create, and the more CO2 molecules can transfer into the water! Ideally, the CO2 bubbles should never reach the top tank surface. That would indicate that all of the CO2 is transferred into the water. In reality though, many CO2 bubbles will reach the top of the tank, and that CO2 is then lost.
The other important parameter is the velocity with which the water is circulating in your tank. The faster it goes, the quicker you reach the point of CO2 saturation. There is however somewhere an optimum speed, because the faster the water surface is turned over, the faster you loose the CO2 as well.

Ultimately, you also have to consider the chemical reactivity of CO2. You probably know that a small percentage of the dissolved CO2 reacts with water to form carbonic acid (H2CO3) which then is the starting point of a whole cascade of other chemical reaction. But that is a different issue.

I hope I didn't confuse you to much, as I believe that in the end you really have to determine your bubble rate experimentally, as the bubble SIZE is the most important factor!
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post #6 of 8 (permalink) Old 10-05-2015, 12:44 AM Thread Starter
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Alright, thanks for the responses everyone. Knowing that larger tanks are going to have larger plant loads is what led to the question in the first place. I knew this had to mean more Co2 consumption and that is why it didn't make sense that I was seeing only the 1-2 bpm suggestion.

Okay, here is my next question. I have an Apex that monitors my pH. My wc are with RO/DI water and I buffer with Seachem Acid & Alkaline buffer to ~6.5. So I do not have to worry too much about kH changes in my tap since I'm starting with 0kH and buffering to a desired 4-5kH. The drop checkers are new to the market since I used to keep planted tanks and I understand that they measure pH in as much as they measure the change in the captured air between the solution and the tank water. I assume that the drop checker is preferred for one or more of the following.

#1. It's far cheaper.(Not my issue as I already have the pH meter.
#2. It measures only the pH in the air between solution and tank; thus a better read of dissolved Co2 in the tank because more things can affect the pH in the tank water itself whereas only the escaping Co2 into the drop checker should be affecting the pH there?
#3. Both of the above?
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post #7 of 8 (permalink) Old 10-05-2015, 04:02 AM
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With a PH controller what you want is to take a sample of tank water and de-gas it by running an airstone in it for a half hour or so. This drives all the co2 out of the water and gives you the base PH of your tank, presumably the 6.5 PH that you are mixing when you make your water. To get approximately 30 PPM of co2 in the water you would set your PH controller for 5.5 PH. Your controller should do the rest and keep your PH steady and that means your co2 is also steady.

If it were me I would slowly raise the PH of your de-gassed water to 7.5 so your PH is not so low when you add the co2. Then set your controller for 6.5 PH. A 1.0 drop is what you are after.
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post #8 of 8 (permalink) Old 10-05-2015, 01:18 PM
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lol you have been totally blasted by science.

I run what could be called a stream of bubbles on my 75g. Probably upwards of 6-10 bps (who knows). Also worthy to note that I use a surface skimmer on my tank as well so have quite good gas exchange. Just ramp up your CO2 slowly. It's easy to tell when you don't have enough (deficiencies in plants and/or algae because of too much light) and it's easy to tell if you have too much (fish are unhappy).

Like others said it depends on plant mass / light / rate of consumption blah blah blah but there's no magic number here. You'll just have to eventually figure out what works for you. The 1 pH drop / using a drop checker methods are a good start but certainly don't mean you will be fine if you meet these criteria. Having an Apex will help you out a lot (I also run an Apex and use mine as a 'safety net' floor to stop CO2 if my pH gets too low).

Using a pH controller is good to 'ballpark' estimate how much CO2 is in the tank but then you are subject to exactly what your pH might be. Changing kH levels and water parameters may mean that you read a different pH for 30ppm CO2 one week to the next. HOWEVER because you use RODI water this may not be an issue for you. I just lock in a constant bubble rate now and let it run for the entire photoperiod.

Also drop checkers are useless. pH controllers are 100x more useful. Mine is yellow all the time and everything is fine. I run into issues at lower CO2 levels.

I also strongly disagree with the above poster's idea to raise your pH so that you drop to 6.5 vs 5.5. I don't think this matters at all and don't think you should EVER alter your pH just to hit a certain number.


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