Actually, we hated the national aquarium in Baltimore. What a confusing mess (and noisy) to get around. Wish we had that money back.
Anyway, the Shark Reef at MB is OK. It is pleasantly quiet, dark and cool, easy navigation, enough displays of interest, relaxing time.
As for the cost, I always think of what I would have probably lost in the casino for that period of time. Shark Reef comes out way ahead on that.
If you just want to see a lot of pretty fish, see the fish tank in the Forum Shops. This is free.
And it used to be you could buy a 'tour' with the fish feeders, see how they do it, etc. Don't know if this is still available. Never did it.