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 [tag:multi-pet-families] question* is likely to be interested in, like finding common environmental conditions, pet-on-pet predation, etc.

Some examples:

* https://pets.stackexchange.com/questions/1398/how-can-i-resolve-a-fish-conflict-in-my-aquarium
* https://pets.stackexchange.com/questions/1409/which-shrimp-will-co-exist-with-my-loaches-and-schooling-fish
* https://pets.stackexchange.com/questions/1487/giant-gourami-tolerate-tank-mates

These are all tagged [tag:m-p-f]; there are plenty of similar questions that are not, like https://pets.stackexchange.com/questions/1427/besides-water-quality-what-should-i-consider-before-adding-more-tropical-fish.

I'd recommend we use something like [tag:aquarium-communities] for these instead. The common term for a tank with multiple species is a `community aquarium` 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.