Keep aquarium and fish as separate tags.

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

Keep aquarium and 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 aquarium tag paired with another animal tag (such as frogs or turtles), and there could even be questions with only the aquarium tag and no animal tag (for example, a question about aquarium maintenance or repair).