At aquariums the glass is the main challenge in capturing great sea life, minus the glare that usually hits the aquarium glass, or blur motion if you take the photo without flash. The easiest aquarium photos occur in well lit areas with outdoor sunshine or strong artificial light injected into the tanks.
It shouldn't be hard to get great photos that you can frame or show off to friends if you look for the lighting sources, and pay attention to the details. Faces are normally the most popular aspects of animals and fish, or the interactions between the critters make for adorable shots. Also see if you can catch them while they're eating. Some animals put on quite a show. And finally, there are some aspects of the animal world that are not so appealing. Sure you can take those pictures, but seldom will you use them.