Small fish biting large fish

Question
QUESTION: I have a 28 litre tank and three fan tail fish. I've had one of the fantail fish for five years so it is now quite large. The other fish in the tank recently died so two weeks ago I introduced two small goldfish a week apart. The newest goldfish has recently started biting the fins of the larger goldfish so I put it in a separate tank (although this is only a small tank and I think it would be cruel to leave it in this tank). Now that one of the small goldfish has been taken out of the tank the other small goldfish has now started biting the fins of the larger goldfish. Prior to the biting the larger goldfish was active, now it sits on the bottom and does not attempt to fight back. Please help!

ANSWER: hmm this question confuses me. it could be as simple as you have a bully fish on your hands or that these fish don't get along. a bully fish is a fish who wont stop picking on the other fish. i have a bully fish who likes to chase the fish. is that what happens or does the bigger fish tease the other fish? does the bigger fish have clamped fins? how often do you change the water?
corinna

---------- FOLLOW-UP ----------

QUESTION: Thank you for your reply - I'm confused too! I thought that one of the smaller fish was a bully fish, but when I put it in a separate tank the other smaller fish took its place and started biting the tail of the larger fish. The larger fish doesn't tease the other fish, but hides and lies on the bottom when it is in the tank with either of the smaller fish, yet they still attack it! The two smaller fish don't attack one another. I've now put the larger fish into a separate tank and it seems much livelier, despite the tank being much smaller.

I'm not sure what you mean by clamped fins. The bigger fish is a fan tail with very long fins. The two smaller fish have long fins also and look similar to the larger fish before it grew.

I have a good filter system in my tank so I only change a third or half of the water once every 3-4 weeks. I've noticed that the water is much warmer than it usually is and I've read that this can encourage the fish to breed and that the male fish sometimes bite the fins of the female fish. Could this be the case? I have put the "victim" fish in a separate tank for the meantime, do you recommend that I continue to keep it separate from the others, and how long for? The tank that it is in is not really ideal for it.

Thank you for your help,
Lauren

Answer
well it sounds like your bigger fish needed a vacation. even though its not really the season for breeding this could be possible. around the winter season male fish get white spots on their dorsal fins simply meaning that it is mateing season. if your bigger fishs' dorsal fin is down then she has clamped fins. i would suggest keeping her in there until we sort this all out. maybe raise the water temperature and monitor their behavior before putting the other fish back in.
corinna