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By the time you had enough livestock of any species to generate a serious amount of CO2 these animals would also be producing toxic levels of ammonia.
Marble is mostly calcium carbonate and magnesium carbonate. Coral sand, oyster shells, limestone rocks and similar materials are also calcium carbonate and magnesium carbonate.
When these materials are under water, and the water is acidic, low pH, these materials break down and add their minerals to the water. Levels of calcium and magnesium (GH) rise and carbonates (KH) rise.
Yes, there is a reaction between carbonates and CO2. As the level of carbonates rise some get changed into CO2. But this is not a significant source of CO2 in an aquarium. The reaction is rather slow, and the CO2 off gasses or can be used by the plants so fast that the CO2 will not build up significantly. I know 'gets used by the plants' is the goal, but the amount is so low it almost does not count. In the mean time the GH and KH are rising toward levels that will make the tank a good home for rift lake fish, live bearers and many rainbow fish. This is fine, if these are the fish you want to keep, but if your goal is Tetras, Rams, Rasboras, Cories and many similar fish, the water is too hard for them.
The very fact that the water has a high KH shows that not much of the carbonates become CO2.
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