From their site:
From September 7, 2012, through March 22, 2013, admission to the National Aquarium, Baltimore, is just $12 on Fridays after 5 p.m. Last entry time is 8 p.m., and you can tour the exhibits until 9:30 p.m. Blackout dates apply. Add on a 4D Immersion Film for just $5.
I don't know if this fits your schedule, but it really was the best time to visit, often we felt like we had the place to ourselves.