Safe CO2 level for livestock
I am running pressurized CO2 and high light in my tank. Photo period is 2 sets of 3.5 hours each with a 7 hour siesta in the middle.
Livestock includes tetras, rasboras, SAEs and snails. I find that some of the fishes (All SAEs and a few tetras, rasboras) hang around very close to the water surface during the last 1.5 hours of the photo periods. They seem to be deprived of Oxygen. Not all fishes behave this way. Once the air stones kick in right after photo period, all fishes behave normally.
pH drops from 6.8 to 5.9 or 6.0 during the photo period. Not sure what the fully (24 hour) degassed pH is, but the tap water I use for weekly 60% water change is close to a 7.8 pH.
I am fairly confident that the CO2 level in water reaches to >30 ppm levels during CO2 injection (based on pH drop and CO2 reference and indicator bulbs).
My question is, will the fishes suffer any long term damage because of the seemingly high CO2 levels that last for around 1.5 hours during each photo period?