Shrimp
For most people, shrimp aren't even taken into consideration when setting up an aquarium. They aren't the most visually attractive creatures in the world, but they can actually enhance the well being of your aquarium. They eat the food that your fish miss, which can be an asset when cleaning your aquarium, and are really fun to watch, despite their small size. The rule of thumb is 1 shrimp per gallon of water. This rule, unlike "1 inch per gallon of fish", works quite well.