I would say, in general, there are no LFS' worth visiting in and of themselves in metro range of DC. (I've never been to Tropical Lagoon) I also personally don't think either of the Smithsonian tanks are worth the visit in of themselves, though if you're in the zoo or museum they're definitely worth a stop.
The National Aquarium in Baltimore is nice, but they charge a $40 a ticket, which IMO is very steep. It's been about 10 years since I've gone; I don't remember it being stellar. (that being said, I'll visit again this summer. My recommendations of the ones I've been to: Osaka, Omaha, Berlin. San Diego is nice too). If one went out there by car, I'm told there are 1-2 good stores in between DC and there, but I don't go out to suburban MD a lot so I don't know which ones.
IME, the best LFS in the DC area is Blue Ribbon Koi. They are good LFS, but not worth the visit by themselves for visitors, IMO. BUT, they are like a 5 minute drive from Manassas Battlefield National Park.
Going further afield, SaltwaterAquarium.com is near Gettysburg, about 1.5 hours from DC. (and therefore next to said battle's national park - most of the big name US Civil War Battles, as well as a lot of smaller Civil War History, is within 3 hours drive of DC)
A drive through the Chesapeake Bay area from DC is also very nice and relaxing; about 2 hours from DC on the Eastern Shore of MD is
@PacificEastAquaculture which I've had great experiences with, though I've never visited in person.