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
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