The fish definitely sound like something bacterial grabbed them
Or something toxic.
Though your filter dying could have started a mini-cycle that is not likely since you have to detectable ammonia readings (and none in nitrIte either in a mere 3 days I doubt a mini cycle could finish up. your nitrates though high for my taste are in the accepted zone for most kits.
With the tight clamping of fins, discoloration and depression it point that way.
But they would need to be either exposed or highly stressed where their own on-board yuks can take advantage. Was there a power outtage? Do you think they lost heat and had a temp swing?
Is your tank open topped?
Have you sprayed for anything (even a liberal endusting) in that room? Or a liberal dusting of caerpet fresh?
Are they near an open window? (in some states like parts of FLA, the city or county starts the annual mosquito spraying down the block)
Haveyou tested the tap water (since you just did a change) to see if any large changes have come?) Has your water company sent you any infocards this past 2 months?
Does you tank product cover chloramines?
ANd read your box..does you NH test contain Nessler's reagent?
I know..a lot of questions... so little time... but an answer to one of these could speed up advice in the right direction.
I'd pick up some Marycin2 form petcomartworld and start treatment for the fin rot and discolorations