Give him your support, which you already do. Go out with him when he shops for aquarium stuff. I can't tell you how many guys love it when their significant other helps, just don't point at a fish and say "I want it". This way you both win, I have seen a few relationships break up because of tanks.
A BWA gift certificate would be the perfect gift.
Kidding.
On boxing day all our gift cards will be $20 more than your purchase price.
Ken - BWA