This post made me smile. When I was a (very tormented) teenager, I had my first tropical aquarium. That aquarium kept me sane, I swear! My mom would find me sitting there watching the fish at 2:00 in the morning. While I don't do anything like that now, I still find that staring into one of my aquariums can calm my nerves (unless something is wrong in there--then I'm glad to notice it). I bet I spend an average of 15 minutes a day just sitting and watching one or all of them (I have five running currently--yikes). In addition to learning about what is normal and not normal behavior in your fish, one learns a great deal about nature from watching an aquarium. It also alerts you to potential problems.
Enjoy yourself! Tell your girlfriend that you are doing biological studies.