Api test kit to check ammonia, nitrite, nitrate and ph, do a 30% water change, make sure you treat the water you're replacing with prime or a good water conditioner if you can get prime I would highly recommend it if you don't have it already, the api master liquid test kit you can get accurate readings where strips do not, the api master liquid test kit is a bit pricey but in my personal opinion it is an absolute must have, make sure the water is temperature matched abd treated before putting it back into the tank, it could be fish tuberculosis but bad water parameters can also cause it so test your water, do a 30% water change treated with prime if you have it if not use a good water conditioner turn the lights off to reduce stress from the water change.