I just joined, so this isn't very timely but could be helpful in the future.
There are no pet shops in Concord that are privately owned, just chain stores.
Petco, Ft. Eddy Road. I-393 exit 1, right off the ramp onto Ft. Eddy Rd. At the 2nd light, turn right, and left at the stop sign. Follow around the front of the bldg.
PetSmart, Loudon Road. I-393 exit 3, right off the ramp onto 106, and right at the first light onto Loudon Road. Right at the 2nd light (across from Wal*Mart) and left at the first intersection, on the right.
I used Petco for years, but was never really happy with them due to their fish and plant quality and selection. Once Petsmart opened I have been going there since except for a couple of occaisions.
I hear that Wal*Mart has a fish section, but I can not confirm since I don't do Wal*Mart.