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.