Keep [tag:aquarium] and [tag:fish] as separate tags.

The consequence of this is that likely the vast majority of fish questions will have both tags.  However, there will be some questions with the [tag:aquarium] tag paired with another animal tag (such as [tag:frogs] or [tag:turtles]), and there could even be questions with only the [tag:aquarium] tag and no animal tag (for example, a question about aquarium maintenance or repair).