One of my goldfish, which I've had for 3 months, suddenly got a white patch on the top and a white spot on its left side (the yellow-black one in the picture). I just noticed it today, but I'm sure it wasn't there yesterday, so it happened very quickly.
I just moved my fish to a new, larger, 20 gallon (76 liters) tank 10 days ago, and added a Seachem Flourite substrate, but other than that everything is still the same.
Any help is appreciated, thanks.
UPDATE: the white film on the fish seems to be spreading, I see it on its head, gills, and tailfin
I also noticed brown spotting on its fin and from researching online apparently it might be from ammonia but I've been doing 10% water changes almost daily, and none of the other fish appear to be affected, anyway I did a 50% change today since I didn't have any water testing strips lefts just to be safe.
but the thing is you can only see it under a bright light, when it's not directly under the LEDs you can barely see it