I have six Serpae tetras, five rainbowfish, and one Cory cat in a 25 gallon community tank. It is heavily planted. The rainbowfish are a new addition.

The tetras have been fin nipping and killing other fish. So far they have killed 4 fish, one of which was even there before it was. Three dwarf neon rainbowfish and one Cory cat.

How can I reduce this aggression and fin-nipping from the serpae tetras, and provide a safer home for my remaining Cory cat and new (regular size) rainbowfish?


I have 8 serpeas in a 5 gallon tank. At first I had 4 and then they were very aggressive. Then when I added 4 more, it was only one that was aggressive. When I removed him, none of the others where getting hurt.

You should remove the aggressive ones then that should stop the nipping. I hope this helped!

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Serpae tetras tend to be less aggressive if kept in a larger group, like more than 6-8. This does not ensure the occasional nip at a peaceful cory. Kindly do a bit of research before grouping together fish in a community tank. There are a number of online resources to find out compatibility between different fish.

A community aquarium needs fish which can tolerate a similar range of water temperature, pH, and gH/kH. Once this particular need is matched, you need to know about aggressive fish species. Corydora catfish need non aggressive tank-mates, and they MUST be kept in groups of 6 or more. Cory cats get very very stressed if kept in a lower number.

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Add 6 more tetras. This will increase your tank's population and thus reduce the new fish you can add, but unless tetras are in groups of at least dozens this will happen. Additionally try to increase frequency of water changes and when adding new water try to have it one degree colder than your tank water. Tetras are the type of fish that love rain and feel more calm after a rain. As a matter of fact tetras in the wild spawn only after good rain. Your tank is already planted, so you are covered there.

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Serpae tetras are notorious fin nippers. There's really no way to prevent a fish from doing what it does unless you separate it. See if you can return them to where you purchased them or possibly sell or give them to someone who has a tank that can handle their temperament.

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  • There are methods that I am having success with. – Patrick Sebastien Oct 17 '14 at 16:14
  • @PatrickSebastien Please explain if something other than separation is working. – Jestep Jan 16 '17 at 22:09

Serpae tetras are by nature fin nippers. They are only second to tiger barbs. Although I always wanted to have them again, I restrained myself knowing what they are capable of. It is just their nature. The docile Neon tetras can become aggressive to each other if you have fewer than half a dozen of them.

I've seen aggression in Neons in my small 5 gallon tank where I kept 1 betta, 4 neons, 1 cory and 2 black neons. The aggression stopped when I add 2 more neons. My 5 gallon tank is mainly just for the betta and the tetras are just tank mates. Bottom line, if a normally docile neon tetra becomes aggressive with few numbers how much more to a naturally aggressive Serpae. Mind you my neon tetras came from a larger population in my larger tank.

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