You know you're a fish keeper when you have varieties of live worms in your fridge and all your other housemates totally freak out when they see worms in the fridge.
Another one: when 3/4 of your paycheck goes to the mom/pop LFS and you may well be the sole supporter of that store... and you can barely afford groceries because you would rather have more fish/supplies
You know you're a fish keeper when you start planning to buy a new house so that you have room for all or fish tanks.
You know your a fish keeper when you meet someone and the first words out of your mouth are "so do you have a fish tank"
You know your a fish keep when you literally had to move your bed out of your bedroom to make room for tanks.
You know you're a fish keeper when you wonder if you'll have enough room for a nursery for your new baby and then decide the kid can sleep in a bassinet in the living room.