when you find yourself bringing home a new betta, and hiding it until your significant other is out so you can just sneak it onto the shelf with all the others
or
when you buy new fish when you know your significant other is out so that they won't roll their eyes at your because you've brought home even more fish
when you're buying a new house, and you look specifically for floor plans that will allow you to have a fish room
when you have a whole shelf in the closet devoted to spare fish equipment
when you find yourself giving customers at the pet store fish advice, and you don't work there
when you start eyeing places in the house and think "i could fit another tank right there", and "who needs a dining room table anyway, think of the tanks i could put there", and "hmmmm, what size pond could i fit out on that patio and would it take the wait" when you live in a second floor unit