Fish freak out and gasp when light is turned off HELP

[MeatGoesHam]

Hello guys first time post here,

Recently got a 10 gallon tank with a hang on back filter, heater, thermometer, and an air pump. I have 4 Pretty Tetras and recently 2 Corydoras. When i turn off the light the Prettys all swim rapidly at the surface of the water and gasping. The Corys on the other hand have not acted this way since i've got them. I thought it was due to ammonia/ low oxygen but i test everytime and it does not come up to even low levels of ammonia and I have the air pump for oxygen now. I have also tried turning on my room light and keeping it on for a period of time after I turn my LEDs off but still they do this.

Any solutions/answers to my problem?

Thanks in advance

[jojo]

Is it a planted tank? It could be CO2 poisoning.

[MeatGoesHam]

I do have a planted tank so maybe but I don't have CO2 injection. I got a moonlight LED since and it seems to have remedied the problem but thanks!