You could keep Rocks and you could maybe put a little treasure chest inside. OH YEAH and you could put a mini shipwreck in the bottom so the fish can swim inside it. if you are wanting a creature you could put a crab or a terripin in it =]
Apart from fish, snails, or any other ground dwellers?Well there are the African clawed and dwarf frogs.The Mexican walking fish, or axolotyl, is an aquatic salamander, with gills, that are very interesting.Several species of shrimp will live on the plants in the aquarium, but others will stay on the bottom.Marbled crayfish and Cajun dwarf crayfish are fascinating crayfish, but again would stay on the bottom.Aquatic turtles are fun to watch.Dragonfly larvae live underwater on the plants, but need live food, preferrably fish fry or small fish.Mosquito larvae are pretty active little critters.In a marine tank, you could keep starfish, crabs, and shrimp.* And anemones.