It also counts for something that some things are good in moderation. Like if your gf wanted to turn the whole house into a pottery studio and start hosting classes all day, you might be a little offput. Same goes for her with your fishtanks, a few is great but when it dominates life and home its not so great. Also note that alot of these guys do have fish still- theres a difference in saying "i'd rather not have 10 tanks" and "you have to get rid of your fish, because you love me more then your hobbies right?". Usually they're not trying to change him, just that, not being fish geeks, they'd rather not have the fish start migrating in to every room of the house. It is their house too, after all.