FISH DYING AND BREATHING HEAVY

[Nate]

I'd put money on you having an issue with a protein film on top of the tank. This prevents oxygen exchange and tends to happen in mature systems running a cannister filter (205) and no skimmer ( or a cruddy skimmer)

It's not the powerheads.

Take both powerheads and point them at the surface of the water, from About 3 inches under the surface to agitate the waters surface and break surface tension .

3 hours later, when normal oxygen levels are reached and maintained, fish will act normal