Before the pets proposal entered private Beta followers of the aquarium proposal were asked to join as all their questions would be on topic here.
There are a few questions about aquariums or ponds I can imagine that are not related to creatures living in that environment. While a question like "How do I know when I need to replace the charcoal in my aquarium filter?" indirectly affects the pets living in the aquarium, there could be questions that are solely about the container or the decoration (maybe even about aquatic dioramas, completely without animals).
As I imagine those question would have been on topic on an aquarium.SE how do we want to treat such questions on pets.SE?