Marine museum or fish tank (8)
I believe the answer is:
aquarium
I believe this clue is a double definition.
'marine museum' is the first definition.
(synonyms)
'fish tank' is the second definition.
(synonyms)
(Other definitions for aquarium that I've seen before include "Tank for aquatic life" , "A kind of fish zoo" , "For live fish and underwater animals" , "Fish tank or marine museum" , "Building containing live fish for display" .)