Another method is to tell him that instead of setting up a new tank, you want him to rearrange all the furniture throughout the house, heavier the better. Then follow up by one week of cold shoulder...
Oh you should be fine. Way worse to have a wife/GF that is like that much harder to change their mind. I'm sure you could make his life a living nightmare or change his mind easier... Or as stated get a new BF
Not compromising on the hobby has made it sooo much better had a fiancee that said 100g is to big then had a fiancee that said softy tank only now i finally am happy with my 250g sps reef.... Dont settel
I think you should ask him for the 100g tank for your b-day. Guys have a very difficult time coming up with b-day gifts, so he will probably buy it for you because he wont be able to think of anything else to get. It's a win-win. BTW, all the other advice on this forum is terrible!