My girlfriend, soon to be wife, at first thought okay .... a fish tank in the family room, that's cool. She moved in with me around that time.
Set up a simple 55 gallon tank with the basics.
Of course then came the "need"
for a protein skimmer, more live rock, bigger clean-up crew, new lighting, corals, another fish, more corals .... you know the drill.
So, I set up a 30 gallon long for her. Her tank. Although I do all the maintenance on it, she gets to make decisions on what goes in it, and we work on it together. That really seemed to help out in the begining.
Of course then came the "need" for a larger tank. So the 55 comes down and the 75 goes up. She gets the 55 for her freshwater angelfish. Her tank again.
I get the 75 for me.
She thinks I'm nuts sometimes, but then again she said to me "it's your hobby, you're home either working on the tanks or talking with saltwater friends online, you're spending a lot of money but you could be spending it on other things, and running around getting into trouble."
I said to her one day, that it never ends - that this hobby is part fish/coral keeping, and part tinkering and experimenting with hardware and different methods.
So she has learned to accept this interest of mine, sees me enjoying it, and knows I love it.
Pretty understanding woman.