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I have a lovely tank(36g) which has been set up and is running beautifully. Tank has 2 rainbow danios, 3 headlight tetras, 2 swordtails, 2 tiny sharks, and two columbian red and blues in it. They all seem fine, colorful, active and happy, but two days after adding the red and blues, one is missing his tail fin completely. when i got him, he had a bit missing...now overnight completely gone? Not ragged but pretty cleanly taken off, no evidence of any disorder, sickness or problem with the tank or other fish...do I have a tail biter in the tank....is it more likely to be the tetras? thanks for any replies or ideas...

The danios and the tetras are known to be a bit nippy, it's most likely one of those. If you have the space, your best bet is to gain more of the same to keep the agression inter-school. When you have more of the same type of nippy fish, they'll tend to spend more of their energy establishing a pecking order and showing off for each other rather than picking on other fish. Also, if it already had signs of abuse, it will be labeled the weak one and will be the first to get picked on.

Good luck
Tim

Not knowing what kind of "sharks" you have, I would probably be guessing that one or both of them is the culprit.

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There is always a bigger fish...

And I highly agree with Tim there about the fish being tagged as a weeker or injured fish, and thus getting picked on. No matter who the culprit is.

One other thing, watch closely for any fungus or fuzzy looking growth on the injured fish. As while the tail is damaged it is now vulnerable to bacterial and fungel infections. I would QT the fish if you are able to.

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There is always a bigger fish...