Tropic Isle in Framingham is the best aquatic store I know of in the state of MA...they are semi-pricey but then again, just watch for the deals...also, online coupons for discounts on all sw fish, all the time, no exceptions - (eg, $8 off of $25 purchase....all the way up the price ladder). I've had great experience from there, their selection is phenomonal and they take great care...also knowledgeable.
Uncle Ned's Fish Factory in Millis is great quality wise but extremely costly.
Lovely Pets in Braintree is my most recent find. At first glance, their selection is pretty vast, prices run on the higher side but I was actually there today and was very distrurbed to see ich in numerous tanks and TOO many fish with open sores, bacterial infections etc. I was really disappointed and left...will not go back.
Lastly, for RI .... 'Something Fishy' in Warwick is pretty good all around.
I personally stick with Tropic Isle...I'm more than an hour away but to me, it's well worth it in the long run.
Hope this helps
Good luck