How long has this tank been setup ? How big is the tank ? How many fish total did you have ? Do you quarantine all of your fish ? You say your water is fine but with nitrates that high clearly indicate it's not, what are you using for filtration ? How often do you do water changes ?
It looks to be either crypto or marine velvet.
Here's some good info on Amyloodiniosis aka "Marine Velvet" and check to see if the symptoms are similar to what your fish are going thru:
Marine velvet