If your fish has numerous small white bumps all over it, it probably has Ich, or white spot disease. See https://www.petfish.net/kb/entry/12/163/
NOTE: In goldfish, small tubercles can form on the head and gill plates. Do not be alarmed, this simply means that it is ready to breed.
If your fish seems to have a grayish fuzz over him and is really lazy, specifically bettas, it probably has Velvet
If your fish is growing some sort of cottony substance out of any part of his body, it probably has Fungus.
If your fish has worms sticking out of it, it most likely has Anchor worms.
If your fish’s fins are shrinking and the edges are turning white, red, or black, it most likely has Fin rot.
If your fish’s scales are sticking out and it’s whole body is bloated, it probably has Dropsy.
If you fish is swollen near it’s anus, it is most likely Constipated. https://www.petfish.net/kb/entry/12/332/
For further diagnosis, see our Disease Articles Category https://www.petfish.net/kb/12/