Just addressing the point about aquariums: I've seen it used for questions about keeping multiple fish species in one tank, and I agree it's not clear or helpful there. These questions often deal with topics that I don't think someone with a multi-pet-families question* is likely to be interested in, like finding common environmental conditions, pet-on-pet predation, etc.
I'd recommend we use something like aquarium-communities for these instead. The common term is actually 'community aquariums' so that might make more sense as a tag, though it might be more prone to misuse on questions where the species mix isn't really relevant.
*For the record, I think the tag is too ambiguous in general. It's not clear to me whether it's describing pet+pet interactions or the human-family side of having pets or what.