Aquarium acquisition (5)
I believe the answer is:
tetra
'aquarium acquisition' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tetra that I've seen before include "Denoting four (prefix)" , "rarely seen flower" , "This indicates a group of four" , "Brightly coloured aquarium fish" , "Swimmer" .)