Fish tank buildup (5)
I believe the answer is:
algae
'fish tank buildup' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for algae that I've seen before include "Simple plants, such as plankton" , "Water plants with no true stems or leaves" , "Seaweeds etc" , "Water plants seen in a gale" , "Marine growths" .)