AC 500 killing my fish left and right

I noticed when you posted your water test results your temp is 80^F. This seems pretty high and mey be stressing your fish out. Danio are a high metabolism fish - they burn/consume a lot of oxygen and warmer water does not hold as much O2.

It also sounds like this has contributed to a parasite infestation as well - with regards to the flashing behavior you observed.

I agree with theory that the fish are getting weak or sick before getting stuck to the filter.
 
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