Fin biting and other problems! Please help

Question
Hi Adrian,

I do hope you can help me! I am new to keeping fish and have had this set up since late November. I have a 10 gallon tank with a black box filter and aerator and 1 hardy plant. I am currently out of pond weed but there is usually some in the tank.

I have 3 shubunkins and have had a few problems with the fish from the off - mostly from what I understood to be new tank syndrome.

They seemed to be improving until recently when the larger fish (I call him Willy) started to bite both of the smaller fish (Wedge and Colin). This went on for a while with a few chunks missing from Colin's tail especially, and eventually they all got fin rot (cotton wool disease). I have treated this and they are in day 4 of the treatment. Things have greatly improved in this respect, ie no more visible bacteria. However, Willy is now a very aggressive fish who has not stopped biting the other 2, especially Colin, for the last 36 hours. Colin is now a nervous wreck floating at the top of the tank with the best part of his back fin bitten away. Every time he comes out of the corner he is getting attacked.

I have just taken the decision to remove Willy from the tank and he is now in a 2 gallon bucket with its own aerator.

I would really just like some advice on what i should do. Before they were getting on fine and I dont know what has caused the change. I have tried to read up on the net but it only mentions agression across species, whereas these are of the same species.

Any help you could give me would be greatly appreciated. I really care for the fish and would hate to lose them. I am quite stressed by the whole situation as I hate to see Colin getting bullied.

Thank you in advance for your help,

Aaron

Answer
Hello Aaron,

Sorry to hear that you are having some trouble with your fish.  Lets see if I can help.  You have not told me the size of the fish in your tank, but since this aggression is going on, I can only assume that they are bigger in size...like 3 or 4 inches.  Please correct me if I am wrong, so that I may make other suggestions.  Assuming that these fish are of that size, some of the gold fish species tend to be territorial...yes, even with their own.  Another aspect is, that if a fish is sick, it will be picked on or harassed by other fish.  You say that you have removed the aggressor from the tank.  You need to treat the other too with a bit of aquarium salt...NOT sea salt or table salt, but aquarium salt.  Just follow the directions on the package.  It should say like 1tbs per 5 gallons of water.  This helps with stress, and I am sure the other 2 are probably pretty stressed out.  Continue to treat the other 2 fish for fin rot, even though the symptoms have disappeared and intoduce a broad spectrum anti-biotic to help reduce infection.  What we can not see with our eyes, another fish may pick up on. Keep me posted to what happens.

Sincerely,

Adrian