Depending on what you are looking for I have a couple recommendations, no chains, and on the way back to Boston via the scenic route. Let me know if you want more info.
Temple Emporium, West Rd., Temple off rte 45; no salt ever, healthy fish and plants handpicked by owner.Tanks not on a central filtration.
The Fish Bowl, Milford, rte 101 on Milford oval; will custom order, carries salt and brackish fish as well as nice healthy selection of Africans.
Not exactly Concord but on the way
Cheryl