*** Boston Tropical Fish and Reptile - Although this was a huge breath of fresh air after Big Fish, Little Fish and Petco, my repeat experiences have been negative. I've received what i consider pretty bad advice from the staff. The tanks are pretty clean, but i was appalled last time i was there when the guy running the place lighted up three different times... IN THE STORE. That's right, he was smoking cigarettes in a pet shop. Needless to say, that alone is enough to drop this place a couple stars.
***** Fins - A bit hectic, usually, but i've gotten some great advice and ok deals, and found the staff to be helpful, if not always well-informed. It's a bit further away than the others, which is the only reason i don't go here for most everything. Easily accessible from the Green line (The Brookline Hills stop).
** Big fish, little fish - The first place i found, and certainly a cave of wonder. Unfortunately, it's a dirty cave of wonder, and hasn't been well stocked any of the (few) times i've been there. The black beard algae completely covering most of the show tanks is the real showstopper.
*** Petco - Near to Boston Tropical, and easily accessible due to it's proximity to the Cambridgeside Galleria mall. There are always great prices on fish here, but then again, there are always about three dead fish floating around in the tanks. Good deals on plants if you can identify them yourself.