Sign male put over top of septic tank … (8)

I believe the answer is:

aquarium

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'tank' is the definition.
(aquarium is a kind of tank)

I can't explain the remainder of the clue.

(Other definitions for aquarium that I've seen before include "Fish-viewing tank" , "Large fish tank" , "For live fish and underwater animals" , "A kind of fish zoo" , "Building containing fish tanks" .)

I've seen this clue in The Guardian.

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