It looks likely to be ammonia poisoning.
What size is the tank and what is the filtration?
What water declorinator are you using?
What are the water parameters?
Water hardness and alkalinity GH
I would do a pwc
/ partial water change, back to back about a 25%, then another 25%. The next day another pwc
but more like 40-50%.
Then the next day another pwc
, and keep that going until you can get the water tested or a liquid test kit to verify the water is safe.
Read the article link in my signature, and the nitrification cycle is discussed and additional links are listed to help you with a tank which doesn't have enough beneficial bacteria / BB to process ammonia and waste. It will help you keep your tank safe for fish.