A prefix for four small tropical freshwater fish (5)

I believe the answer is:

tetra

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'fish' is the definition.
(tetra is a kind of fish)

I don't understand the rest of the clue.

(Other definitions for tetra that I've seen before include "Bright fish; four (prefix)" , "Wrongful act (Law)" , "Small tropical fish" , "in groups of four" , "Small brightly-coloured fish of aquariums" .)

I've seen this clue in the Sydney Morning Herald.

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