FISH DYING AND BREATHING HEAVY

[2pts]

If you have a film on top of the water, that indicates to me that the top of the water is still. ( I don't know if I'm right, just trying to rule stuff out to help you )

Try placing one of the powerheads just a few inches below the water suface (not high enough that it sucks air into the powerhead). Aim the powerhead up so it agitates the surface, this will create "waves" which will help with gas exchange as well as add more oxygen to the water, it will also drop the temperature of the water.

Higher temperature water will decrease the saturated oxygen level in the water. Adding powerheads generally raises the temperature.

Also if your salt level is on the higher end, this also decreases the maximum saturated oxygen level in the water.