In most cases, having a fish tank starts out as a hobby. However, I can understand how it can become an obsession. Sometimes, I feel as if I'm starting to become obsessed.
Fish are relatively inexpensive when compared to the costs of many other animals, and with so many varieties and types, it is easy to become obsessive about the hobby. At first, a 20 gallon tank is great, but then you want to keep more fish, so you look to purchase a bigger tank. Maybe a 30, or 55 gallon tank. But after a while, as your fish get bigger, and you get bored of the species you own, and you want to get a bigger, newer tank. With more fish. Or perhaps you want to keep the fish you own, but you want to try a different species that isn't compatible with your current fish.
So you start a second tank with a different species. But maybe you want to try your hand at breeding fish, which then of course may require some type of breeding tank. And then you want to keep some of the fish you bred, so you need a bigger tank to keep those fish, and the cycle starts all over again.
I know a person who loves the hobby and has his own fish room. He built it in his basement and now has over 50 tanks with all kinds of species. He breeds many of them and sells some to fellow hobbyists. To many people, this can be considered some what obsessive. To him, this is what he enjoys doing and this is what brings him joy.
I've also gone to fish auctions. These are usually sponsored by aquarium fish clubs and it features hobbyists who breed their own fish and bring them to auction off to other aquarium enthusiasts. Again, you see people who have several fish tanks, perhaps obsessed by them, but also bring tons of knowledge to share with others.
In my opinion, it's a matter of how it effects your life. If it is something you enjoy, and it doesn't interfere with the rest of your life too much, then enjoy!