Ian, you need to find a way to get her involved. Take her to the LFS and let her pick out a new fish or some other critter.
Fortunately for me, my wife loves to snorkel even more than I do, and I can't get enough of it. So 12 years ago we were at a garage sale and I talked her into letting me buy a 30g tank with a metal stand. That has evolved into a 125g mixed reef with a 50g display refugium in the great room and a 65g shallow local reef and a 25g frag tank in the reefroom (a spare bedroom). She still isn't crazy about them, but she enjoys them. And I let her turn our entire yard (back and front) into sub-tropical rain forest which she maintains. I cut the grass the first Monday after I moved in with her. She came home from work and told me that was her exercise and she wanted to cut the grass. I haven't cut the grass since (15 years) and she averages 3 to 5 hours a week maintaining the other plants and trees.