They may be beautiful, but these fish can be fearsome.They may be beautiful, but these fish can be fearsome.
They may be beautiful, but these fish can be fearsome. | Canva/Getty Images

Aggressive Tropical Fish 2025: Here are 10 beautiful aquarium fish that don't mix well with others - including the Piranha

By David Hepburn

Writer-at-large

Published 12th Mar 2025, 15:32 GMT

Most of these fish species should be kept in their own tank.

From garden ponds filled with Koi Carp to the humble goldfish bowl – the UK is a nation of fish lovers.

The Pet Food Manufacturers’ Association (PFMA) say there are an incredible five million indoor fish tanks in Britain.

Around 10 per cent of British households own pet fish – making them the third most popular animal to welcome into our homes after cats and dogs.

There’s plenty of choice when it comes to adding fish to (well prepared and correctly set up) home aquariums, with pet shops offering a bewildering range of colourful and interesting finned friends.

But you should be aware that while some fish are placid and happy to live in a ‘community tank’ with a mixture of other species, others are predators that really need their own tank or, at the minimum, carefully-selected tank-mates.

Here are 10 of the most aggressive.

In news that will surprise nobody, the notorious Piranha isn't the sort of fish that mixes well with others. There are a number of different species which grow to different sizes, but all of them have razor-sharp teeth meaning you should also be careful where you put your fingers.

1. Piranha

In news that will surprise nobody, the notorious Piranha isn't the sort of fish that mixes well with others. There are a number of different species which grow to different sizes, but all of them have razor-sharp teeth meaning you should also be careful where you put your fingers. | Canva/Getty Images

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Originally from Southeast Asia, the Flowerhorn Cichlid is a stunning fish but its aggressive and territorial nature means they aren't suitable for community tanks. These are fish best kept on their own or with perhaps one other similarly-sized fish of the same species. Luckily they are so beautiful you might not want any other fish.

2. Flowerhorn Cichlid

Originally from Southeast Asia, the Flowerhorn Cichlid is a stunning fish but its aggressive and territorial nature means they aren't suitable for community tanks. These are fish best kept on their own or with perhaps one other similarly-sized fish of the same species. Luckily they are so beautiful you might not want any other fish. | Canva/Getty Images

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Most Tetras - like the much-loved Neon Tetra - are chilled and happy-go-lucky fish. The exception is the Bucktooth Tetra, which has the nasty habit of nibbling scales off other fish, leaving them open to potentially-fatal infections. If you want to keep these you'll probably not want any other species of fish in the tank.

3. Bucktooth Tetra

Most Tetras - like the much-loved Neon Tetra - are chilled and happy-go-lucky fish. The exception is the Bucktooth Tetra, which has the nasty habit of nibbling scales off other fish, leaving them open to potentially-fatal infections. If you want to keep these you'll probably not want any other species of fish in the tank. | Canva/Getty Images

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Not content with being aggressive to other fish, the Red Devil Cichlid is also known for attacking everything from tank decorations to plants. They are tremendously charismatic fish though and a joy to own - just don't expect them to have any finned friends.

4. Red Devil Cichlid

Not content with being aggressive to other fish, the Red Devil Cichlid is also known for attacking everything from tank decorations to plants. They are tremendously charismatic fish though and a joy to own - just don't expect them to have any finned friends. | Canva/Getty Images

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