Aggression in goldfish

Question
Hello, Mr. Coleman, I'm really sorry to ask a question about goldfish because I know you say you're not a goldfish expert, but I think this is more of a behavioral issue...

Hi! I have a very odd question for you, but it is very, very important to me, and my fish.

Okay, so over the summer I rescued two fish from my swim team's coach's office, one comet and one fantail goldfish. They lived in a tiny bowl, with definitely not enough room. However, they always got along, I never had any issues. Right before Christmas, my fantail developed dropsy, and passed away Christmas Eve. My parents bought me an Eclipse 12 for Christmas, which I set up that day. While my comet stayed in its tank until the system was ready to go, I got really excited and planned the (max) 2 new fish I would get. However, when I was still asleep, my parents ran out to Petco and bought 5 (3 new comets, 1 fantail, and 1 oranda) new goldfish for my tank. eegads, while I know this is too many fish, 2/3 of my fish cost a whopping 29 cents each, so I can't just return them.

This all leads up to my question, though. The other day, I looked into my tank and noticed that the fantail had a chunk of its tail missing! At first I suspected the pump, but later I noticed my old comet, who used to live in a much more cramped bowl, was nipping at the fantail. So I removed him, and put him back in his bowl (I had no idea what to do!)

So I guess I really have two questions:

1. Is the "nipping" I saw (it really looked like a dog nipping) normal? Or should I be concerned?

2. If it isn't normal behavior, how should I correct the problem? Do I need to keep the two in separate tanks? because I feel bad isolating my fish from the new fish. (oh, he generally ignores the other fish in the tank)

I've really become very attached to my new additions, and obviously I love my silly little comet.

I really appreciate your time! I hope I've included enough information, I'll be glad to supply more if you need it :)


Answer
Hi KJ,
 This sort of nipping and aggression is perfectly normal for fish.  That is how they figure out territories and space.

 There are several solutions. The obvious one is to separate the fish.  But, if you go that route you will end up with one tank for every fish which isn't very practical.  

 The other solution involves creating what I call "structure".  Basically, if you have a bare tank with no wood/rock/plants in it, the largest/toughest fish will claim the whole thing as his (or hers) and constantly harass the others.  If you put in structure, the largest will claim the largest "free space" but often leave the other spaces for other fish to live in. Of course, the next largest will claim that space and so on. So, by carefully placing structure in the tank, you create lots of "living spaces" and the tank can successfully hold many more fish than a bare tank.  

  When I say structure, I don't mean a single plant in the middle of the tank.  I mean enough stuff such that there really is an obstacle course in the tank. A tank with a LOT of structure is one where you cannot see the back of the tank from the front.  Fish are VERY happy in a tank like that.

-- Ron
  rcoleman@cichlidresearch.com
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