Snails are very sensitive to copper, so watch out for copper if you use tap water in your tank. They also need a decent amount of calcium in their water to maintain their shell health, which can be helped by feeding the snails foods rich in calcium (such as kale, spinach, and other greens), by floating a cuttle bone in the water, or by using additives.
While Mystery Snails can do a good job helping keep the tank clean, they also add to the bio-load of the tank like other living organisms. In that way, a Mystery Snail is just like any other tank inhabitant. They feed from the tank and they produce waste into it.