My daughter has a small 4-gallon aquarium we put together for her last christmas. Our first attempt was a few guppies with some plants we bought at a local pet-store.
The plants had a few tiny snails that ended up taking over the tank and kept breeding until I finally drained the tank, left it out in the hot sun all week, then put in fresh water with some plants I pulled out of a nearby lake.
After the tank had time to sit with plants and I was assured there were no more tiny snails, we bought one fish, one large golden snail, and three freshwater shrimp called "Velvet Blue".
The tank is quite colorful now, and the shrimp are very fun to watch as they skitter around the tank. Apparently they will breed and create a small colony and keep the tank clean. I tried to take a photo of one but I could not get my camera to auto-focus on it long enough: