Popular aquarium fish (5)
'popular aquarium fish' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for tetra that I've seen before include "... Four < , in composition" , "rarely seen flower" , "Wrongful act (Law)" , "Small tropical fish" , "Bright tropical fish" .)
(Other definitions for barb that I've seen before include "Arrow projection" , "criticism" , "Twisted point of hook" , "Kind of thorn or wounding remark" , "Woundingly pointed remark" .)