GreenGiant
Aquarium Advice Apprentice
Hello, all. I'm looking into a pressurized CO2 system for my new 75g tank, and I'm a bit confused about the best way to accurately control the dissolved CO2 concentration. My tap water has a KH of 4 and a pH of 7.6. The water in my established tank has a KH of 6 and pH of 7.8 (measurement taken just before water change). Using Chuck's calculator, I get 3 ppm CO2 for both tap and tank water.
Now lets say I want to go up to 20 ppm. The first issue I see with the pH controller method is this: what KH do I use to determine my target CO2 concentration? The KH in my existing tank rises between water changes (probably partially due to evaporation, and there may be some carbonates in the gravel, which dissolve slowly over time). If I assume a KH of 6, then my target pH is 7.0. If my true KH goes down to 5 (after water change), then if I control pH at 7.0, the actual dissolved CO2 will only be 15 ppm.
Next issue: I looked at the spec for the Milwaukee Instruments pH controller, and the accuracy is rated at plus or minus 0.2. So if I set the controller at 7.0, the actual pH could be anywhere from 6.8 to 7.2, giving a CO2 concentration somewhere between 11 and 29 ppm. Combine this with the KH variation, and the range gets wider.
With that much variability, would it be just as accurate (and cheaper) to put the CO2 on a simple timer, so that it goes on with my lights and then off again about an hour before lights out? It might be a little more work initially to get the right injection rate, but once the plants establish, I would think that a constant injection rate would give fairly consistent CO2 levels.
If anyone out there is an advocate for the pH method, how often do you calibrate or do maintenance on the probe? The ones we used in my chemistry classes were always finicky, and needed constant attention. Also, is there a reasonably priced unit with better than 0.1 accuracy?
This was a long post, so thanks for sticking it out and reading. Any advice would be greatly appreciated. Thanks!!
Now lets say I want to go up to 20 ppm. The first issue I see with the pH controller method is this: what KH do I use to determine my target CO2 concentration? The KH in my existing tank rises between water changes (probably partially due to evaporation, and there may be some carbonates in the gravel, which dissolve slowly over time). If I assume a KH of 6, then my target pH is 7.0. If my true KH goes down to 5 (after water change), then if I control pH at 7.0, the actual dissolved CO2 will only be 15 ppm.
Next issue: I looked at the spec for the Milwaukee Instruments pH controller, and the accuracy is rated at plus or minus 0.2. So if I set the controller at 7.0, the actual pH could be anywhere from 6.8 to 7.2, giving a CO2 concentration somewhere between 11 and 29 ppm. Combine this with the KH variation, and the range gets wider.
With that much variability, would it be just as accurate (and cheaper) to put the CO2 on a simple timer, so that it goes on with my lights and then off again about an hour before lights out? It might be a little more work initially to get the right injection rate, but once the plants establish, I would think that a constant injection rate would give fairly consistent CO2 levels.
If anyone out there is an advocate for the pH method, how often do you calibrate or do maintenance on the probe? The ones we used in my chemistry classes were always finicky, and needed constant attention. Also, is there a reasonably priced unit with better than 0.1 accuracy?
This was a long post, so thanks for sticking it out and reading. Any advice would be greatly appreciated. Thanks!!
