I also have been to the Baltimore aquarium a couple of times but sadly to say I've lived about 30 miles from D.C. for about 17 years now and was unaware of an aquarium in D.C. I'll have to check it out.
Which aquarium are you talking about? If you are talking about the National Aquarium (I think that's what it's called) don't waste your money. We went on Sunday, it's probably one of the worst aquariums I've ever been in. Almost all the glass is dirty so you can't see the creatures anyways.. If you're not talking about that aquarium then I dunno...
There is The National Aquarium in Baltimore, The National Aquarium in D.C. and The Smithsonian in D.C which has a small exhibit.
Baltimore is by far the largest one.