I think I have found my nipper that might have gotten the tail of my blue one. Now how do I make it stop? Do I have to separate it?

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Tank: 29g:guppies, balloon mollies, swordtail, 2 mollies
Hex: opaline gourami
10g 1F. betta

One of the mollies? I found they can be a bit fiesty.

As far as I know your only option is to seperate it, or see if the fishstore will take it back or give you credit for it.

The only other thing I can think of is that once I had a tank with a rainbow fish in it, he was big, but very peaceful. When he died, suddenly one or two fish that were supposed to be semi-aggressive started getting mean. Evidently the rainbow being around had kept them docile. Not sure what you could put in that would be big enough to keep order, but small enough for your tank though, and not a nipper itself.

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Matthew
30G AquaClear50: Lightly planted, platies, ottos, kuhli loaches.
10G Sponge filter: Heavily planted, ottos, red cherry shrimp

Sometimes having more of the same species can help because they will spend more time bickering and courting with each other, less time chasing other fish. I can't remember the size of your tank though, do you have room for more?

I have a 30G FW.

The nipper is a guppy, he is the biggest of them all. The blue one was the 2nd biggest. Since guppies are fairly cheap, I can get a couple more.

Currently I have 5 Male guppies, 3 balloon mollies (1m 2f), 1 female betta, 2 ghost shrimp, 1 golden mystery snail.

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Tank: 29g:guppies, balloon mollies, swordtail, 2 mollies
Hex: opaline gourami
10g 1F. betta