I assume you're asking whether the phrase "called blue sky" is a relative clause. No, it's not. In fact, it's not a clause at all. To be a clause, it would have to have both a subject and a predicate. As you point out, a relative clause begins with a relative pronoun (e.g., "which"). That pronoun serves as the subject.
"Called blue sky" is a participial phrase directly modifying "aquarium." It works in the sentence more or less like an adjective, except that we find it after the noun it's modifying instead of before.