Need help with hard breathing.

My fish have started to breathe hard, but it only seems to do that when the light are off. Should I put a air pump and only run it when the co2 and lights are off.

Only when the lights and c02 are off? Do you do anything else when turning everything off for the night? Maybe close the lid on the tank? Turning off the fan in the room thus raising the temperature in the tank and making breathing harder? Just some ideas.
 
Do you have a LOT of plants?
Plants produce oxygen and take in co2 during the day, at night, they release co2 and take in oxygen.

A bubbler would help a lot.
 
At night CO2 gas-off would not be a problem for plants, but it would waste your CO2.
 
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