Answer:
Phrase.
Step-by-step explanation:
A phrase is a group of words that that can mean any idiomatic expression or express a concept about something or anything. A clause, on the other hand, is also a group of words that contains a subject and a verb and can sometimes be taken as a full sentence.
"Inside the fish tank" is a phrase as it not only has no verb in it. But it also does not form a complete thought, thus not valid to be a complete sentence.
Thus, the correct answer is a phrase.