They belong in any room where you've got good access to plug sockets, they won't get knocked, are not in a high traffic area and away from large windows. Every bedroom is different, I had 6 running in my bedroom at one point (my bedroom is the largest room in my flat as it's meant to be a living room), often got woken up by buzzing filters, not very practical.
So now I have the quietest one in my bedroom and a noisier one in my tv room where buzzing won't bug me.
There's no set room where a fish tank should be. Some people hate the sound of running water when they sleep, not practical in a bedroom!