Before you go to an aquarium, studying up on some of the different species which will be roaming around could prove to be helpful. Read the following guide to distinguish some of the freshwater creatures.

According to wikipedia, there are literally scores of freshwater fish found in aquariums. Here are some of the varieties:
You are certainly not going to find each and every one of these species at your local aquarium, regardless of its size. Some of these sea creatures might be better suited for a personal aquarium at home, as opposed to a commercial aquarium.
If you really have your heart set on seeing one of these species of freshwater fish at the aquarium, give them a call before you go in order to ask if they have this particular type of fish on view in their aquarium tanks.