The most common and most likely answer to this crossword clue is the 8 letter word AQUARIUM
I've seen this crossword clue in The Daily Telegraph
A
Q
U
A
R
I
U
M
More clues leading to the result AQUARIUM
- Where to see all sorts, from angels to zebras?
- Sign almost marks where submerged activity may be seen
- One immersed in pale blue liquor tank
- Location of underwater activity miles after sign for docking
- Water odd one enters in marine museum
- Sign male put over top of septic tank …
- I put in spirit after water in tank
Buddy explains!

The straight part
We know that "tank" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case tank = aquarium.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does AQUARIUM mean?
AQUARIUM noun- A tank, often made of glass, for keeping live fish or other aquatic animals.
- A public place where live fish and other aquatic animals are exhibited.