Fish tank buildup (5)
I believe the answer is:
algae
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">fish tank buildup</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)