Aggressive fish

These are the species that wreck a tetra tank. Some are fine as the only fish. Some need a species tank of their own kind. None of them become community fish because the shop kept them next to guppies.

Oscars want a bathtub. Rainbow sharks want one fish of that type per tank. Mbuna want rockwork and their own water chemistry, not a planted 20. Male bettas are on this list because they fight other males and nippy tankmates — they still make sense alone in a 10.

• AngelfishSemi-aggressive · 29 gal min

  A cichlid that people treat like a tetra. Adults eat neon-sized fish and shrimp. They want height more than length — a 29 is the floor for a pair.

• BettaAggressive · 5 gal min

  A 5-gallon heated, filtered tank is the floor. Males fight males. Long fins get nipped by danios and barbs. Some keep shrimp; many lose shrimp.

• Clown LoachSemi-aggressive · 75 gal min

  Sold at three inches, grows past a foot, wants a group and a 75-gallon. A classic “I did not know” fish.

• Convict CichlidAggressive · 20 gal min

  Tough, cheap, and mean once they spawn. Fine in a species tank. A disaster in a community of tetras.

• Electric Yellow CichlidAggressive · 30 gal min

  Mbuna. Hard, alkaline, rocky tank with other Lake Malawi cichlids. Not a planted community fish, no matter how pretty the yellow is.

• Fiddler CrabSemi-aggressive · 10 gal min

  Same story as red claws: brackish water, land, and a species setup. They do not live in a 10-gallon community.

• Jack DempseyAggressive · 55 gal min

  A large Central American cichlid. One adult wants a 55-gallon and tankmates that can take a punch — or no tankmates.

• OscarAggressive · 75 gal min

  A pet, not a community fish. 75 gallons for one, more for a pair, and a filter you will actually clean. They know who you are.

• Rainbow SharkAggressive · 55 gal min

  One shark per tank, and not with other similarly shaped bottom fish. They own the lower third of a 55.

• Red Claw CrabSemi-aggressive · 10 gal min

  Brackish, with a dry beach. They will pick at fish and each other. This is not a community invertebrate.

• Tiger BarbSemi-aggressive · 20 gal min

  Fin-nippers unless they are in a big school in a big tank. Not for bettas, guppies, or anything with trailing fins. Six is the minimum, ten is better.