Help!! Emergency

It ain't nitrates , def a fish disease and if you ever worried of nitrates I would get some marine pure and put it in the sump that cheeto bio pallets and good skimmer can take a high 100 plus to 20 in a week or so cough cough not mentioning my name )

 

I'm on the same track as Dave. This is suspicious of contamination in my opinion. If that is the case, that could possibly explain the spike you're seeing in wastes as dieoff occurs.

It could be something has fallen into the tank, or household cleaner in the air, to those plug in air fresheners. These are just suggestions to think about but all can have this effect on your tank.

Are all your heaters intact and where they should be?