I don't know of any within the city limits of DC, but there are some awesome ones outside of it. What are you looking for? Coral, Fish, Clams? All of the above? lol
If you happen to go to virginia, the store Marine Scene is pretty amazing. And in my area is Aquarium One which has a lot of corals/frags. If possible go check out the Baltimore Aquarium, it is really great. I hope you enjoy D.C.!!