My fish named Harvey has black stuff on it's fins, I didn't buy him like that.?

Question

I don't know what it is, but i think that it might be the reason my other fish twitch, it attacking him constanly!

well not really attacking, chasing is more like it.

Answer 1

It sounds like a fungus,my fish had that and if you go like to petsmart or something go to the fish section, and talk to a worker,they should give you something to put in the water.and the other fish are attacking him,put him in a separate tank

Answer 2

Fin rot and fungus are almost always white/translucent in apperance, not black.  Black fins are either a sign of healing or damage, likely from ammonia burns.  If you test the water and find 0 ppm ammonia and nitrite, I would guess it's just recovering from having the other fish bother him.  Goldfish will really only bother other fish if they feel threatened or cramped-- if those are Common Goldfish you're talking about, you should be looking at a 55+ gallon tank, and about 25 more gallons for additional Common.  For Fancies, I'd start at 20 gallons, and add 10 for each next Goldfish.  Keep up with weekly (or twice weekly) water changes and make sure your filtration is superb.

Answer 3

sounds like fin rot. You can buy medicine from the pet store to cure it. It is caused by dirty water. Have you cycled the tank? Do you do water changes every week??