Hey, I just moved to Allston, and I was a bit surprised to find that there aren't many lfs
around. I've only been to three so far, and the first two I wouldn't personally recommend. The first place I went was Petco, since it's a couple blocks from my apartment and I could walk there. Big mistake, I had a very unpleasant experience with the staff trying to tell me blatantly wrong information about filters and beneficial bacteria and I will never go back there. Second place I went to was Big Fish, Little Fish in Somerville. The guy at the counter was pleasant enough, but the shop was pretty dirty and they didn't have a large selection.
The last place I went, and will go to exclusively from now on, is Tropic Isle Aquarium Inc in Framingham. It's right down I-90 from Boston and has a HUGE selection to choose from. On top of that, the staff was very knowledgeable and very helpful. That being said, they definitely let me wander around taking in the livestock for a bit before they approached me, so don't be worried if they don't pounce on you the second you walk through the doors. They have a ton of common fish in there, but they also have some really cool ones that you aren't going to see at places like Petco. Just remember you can always come back after you research a fish if you find something you've never heard of and really want it
I may also have wandered around the store for 1+ hours before I was able to even think about asking for help getting fish