Monster is totally worth it, there are also a few stores in the chinatown area that are pretty big (one of them closed down I think though, I forget which one).
Fishtown is alright. I did get a bunch of cichlids there back when my cichlid game was stronger.
There was one on Mott that closed down, it was pretty awful tbh and I won't miss it. And there's another tiny one on I think eldridge or something, but it's not worth your time.
Pacific Aquarium on Delancey (in/near chinatown) is a great store and I go there for all my local needs, but they don't have very much that's rare. The rarest thing they've had, which they've had for some time, is emperor cichlids. But they just moved them to a display in the back that was too small and they beat each other up so they're in terrible shape now.
But annnnyway, that's a pretty good shop.
Also highly highly recommend Fauna on the upper west side if you're interested in reptiles, amphibians, birds, or SW. Their FW section is pretty small.
There's another SW place right next to the Javitz Center but that spot is nearly exclusively salt (I think they had one african cichlid tank) but is very very impressive.
