Since I consider it too late in the day for a visit, I make a general inquire about the admission prices just in time to stumble on an October Surprise.
Admission is only $5 after 5pm every Friday in October with the Aquarium staying open until 9:30pm on Fridays.
It takes at least 2 hours to view the aquarium and you will be given a set entrance time.
I was able to purchase my ticket for a 4:30pm admission and had no wait time. However, leaving the National Aquarium around 6:30pm, I discover there was an extensive line for admission. I recommend you get there early to avoid the line and it might just be the best $5 you have ever spent.