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I have a 60 l freshwater aquarium. After water changes I see plants producing gas, which raises to the top just like when I had CO2 fertilization running (currently turned off since some weeks). The production goes on for up to 2 hours and seems to correlate with the amount of water changed.

I assume it's oxygen since I don't know of another gas produced by plants. Reasons for it to might be the high O2 saturation of the fresh water by the faucet aerator of the tap or the shower head, but it's unlikely since I collect cold water before warm water from the shower in a bucket which remains calm for up to 4 days.

What could be in the freshwater to be temporarily present or what could be partially removed from the old water that causes the activity inside the plants?

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The plants will produce oxygen which normally goes into solution. However, if the water is well oxygenated, not all of the oxygen will be able to go into solution, and gas bubbles will be created. As the dissolved oxygen comes out of solution (e.g. overnight), dissolved oxygen levels will drop, and oxygen produced by the plants will again be able to go into solution, and no bubbles will be seen.

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