I am in Springdale MD, close to Largo MD. Have you tried RD fish and pets in Waldorf? Totally Fish in Silver Spring is very nice. Showcase aqaurium and also Tropical Lagoon in Silver Spring. Riverdale Pets is also nice.
Oh yes I've been to RD, nice guy who owns the place
There is a place in Clinton called Roozen's the front is a nursery but the back is tank after tank of salt water fish a couple of large vats for live rock.......kinda primative but all in all not a bad place if you are careful of your selections I've gotten some nice stuff there, they have a web site http://www.roozens.com/
There is another place also that deals in custom salt water tanks in Fort Washington on Indian Head Hwy called Ultimate Saltwater Aquariums http://www.usaquariums.net/ they have an awesome selection of corals and fish, very clean exceptional service