Went during the week, no wait in line. We had the aquarium almost all to ourselves.
Beautiful fish that you can view at your own pace. An afternoon well spent, with or without kids.
If you want to save the walk and the casino doesn't interest you, park on the south side of Mandalay Bay in their convention center - it will leave you right at the entrance to the aquarium.
Unfortunately, we paid the parking ticket of $10 and there was no one checking tickets at the exit. Take a chance ? and maybe save $10.