Great point, Matt B. If you hadn't considered shrimp before, they're really pretty fun to watch. I plan on doing the same with an old 2.5 gallon I found in the basement, gonna wash it up really well and put a touch of black sand, some plants, a manzanita branch, and toss in a few blue pearl and sunkist shrimp.
I am not trying to be mean but am using an example.
When most people think of fish they think of aquarium fish they think of goldfish and betta fish. Now when some people purchase these fish they put them in bowls or small tanks. Bowls are meant for plastic plants and cereal. Now on the bright side you said you have a tank. 2.5 gallon. You could put shrimp in that tank and that would be fine. The smallest size tank for fish in my opinion is 5 gallons for a betta fish.