CO2 levels in tank - The Planted Tank Forum
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post #1 of 3 (permalink) Old 04-15-2010, 01:47 PM Thread Starter
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CO2 levels in tank

Can I assume that if my tank water measures a pH of 7.2 and a KH of 6.5 first thing in the morning (lights just on for maybe an hour), that my fish have enough oxygen to "breath"? According to the CO2 chart on the Krib ( that leaves about 12 ppm of CO2. I've been concerned because when I experiment with lessening surface agitation (for the health of my plants, to keep CO2 in the tank), my fish sometimes look like their gills are flipping a bit more than usual. That's what they were doing first thing in the morning, and I just did a water change yesterday so I know the water is clean

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post #2 of 3 (permalink) Old 04-15-2010, 10:13 PM
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Just to let you know, you cannot use the pH/kH/CO2 relationship if you are simply measuring the kH of your aquarium water. This is because the relationship relies on the fact that carbonates are the only buffering agent in the water contributing to kH, which is not the case in an aquarium.

As such, the best way to determine your CO2 levels is to use a drop checker with a known dkH reference solution (i.e. many of us use a 4 dkH reference solution).

As for your fish, you can slowly try to reduce the surface agitation (i.e. gradually, over the course of several days). That way, they will get used to the higher amount of CO2. Keep in mind that CO2 and O2 can exist separately in the water; having more CO2 in the water column does not displace the O2 that is already dissolved in the water.


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post #3 of 3 (permalink) Old 04-16-2010, 12:59 AM
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Without knowing how big your tank is, whether you use pressurized or DIY CO2, and what lighting you have, I can't see a good way to answer your question in any detail. In general, having some surface rippling is good for the fish and plants. The fish, because it gets more dissolved oxygen into the water, and the plants, because it is safer to use a higher CO2 concentration then.

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