I can't really help because my wife is just about as fish crazy as I am.
She tells me "we don't need any more tanks, this isn't the army" but if I present my reasoning and explain why I "need" this tank she will usually cave.
Unfortunately I'm not sure what will help your girlfriend see that this is important to you, because that can be a big issue: she just sees it as clutter or a nusance and doesn't acknowledge the value you place on your tanks.
Perhaps if you explain that you enjoy it and find the maintenance and observation to be relaxing that will help get her off your case. On the other hand, is she constantly having to be around or maintain aquariums while she is at work? That could definitely be a source of aggravation, having to spend her day around aquariums (especially if there is something about them that bothers her) then having to come home and see yours.