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I drained about 1/4 and removed all decorations and wiped cleaned under running water. Put items back in and filled the tank, using ph lock and safety start.
How much time elapsed after the cleaning and WC, did the fish begin to die?
The described white fuzzy stuff on your fish could be from a number of things, bacteria, fungus even ammonia. Need more information.
I drained about 1/4 and removed all decorations and wiped cleaned under running water. Put items back in and filled the tank, using ph lock and safety start.