I love having snails in fish tanks. Not the large apple or ram's horns, inca, or mystery but the small little round mud snails that are hitch-hikers on plants. These little snails are more an aestic nuscience than anything else, although I've had them crawl into the filter and clog up the works now and again. They multiply like crazy in the right conditions. Some mystaken people believe they are parastites because these snails will crawl onto the body of dead fish and eat them, but they cannot harm live or healthy fish. They also feed on uneaten food and algae, pretty much anything really. They can be a way to transmit disease though since some fish parasites do use these snails as an intermediate host. Just make sure your tank is healthy and in top notch conditions, and that the tank you purchase your snails from is also healthy. Do not purchase snails that have been feeding on a dead fish at your LFS.
For tank cleaning the pointy Malyasian Burrowing Snail (aka livebearing snail) is the best for cleaning and tank maintance help. Mud snails are good, but they do not turn the gravel the way a burrowing snail does. Mud snails will be eaten by puffers and loaches with relish.
~~Colesea