Almost impossible to find in pet stores, these little guys are good for planted tanks, making their meals from decaying plant matter, leftover fish food, and brown algae.
Size - 3/4 an inch to an inch when full grown, some reports of them getting over an inch, but rare
Food - leftover fish food, dying plant matter, brown algae, these guys DO NOT eat living plants. I've watched them clean the brown algae very carefully on my crypt wendtii and madagascar lace without hurting the plant at all. They WILL eat eggs, you do not want these in a breeding tank.
Reproduction - Only one is needed to reproduce, these snails are livebearing. With sufficient food they can reproduce very quickly, though cooler waters may inhibit this to some degree.
Water Requirements - Can live in a wide variety of conditions, though the tips of the shells may become white instead of the usual brown in softer water conditions.
Behavior - Nocturnal. They burrow in the substrate during the day, eating anything they find while doing so, and aerating the substrate. At night they come up to the edge of the water, and scour the plants and decor looking for food.
Goldfish may try to eat them as they are dropped into the tank, and may knock them off the glass for kicks, but are unlikely to hurt either themselves or the snails. MTS have a trapdoor they can close to protect themselves from chemicals and fish. They are exceptionally hardy and can be difficult to get rid of, some reports have them staying alive after weeks in dry conditions, through bleaching and freezing (don't release them into the wild). Boiling the substrate will kill them however. Clown loaches can keep them under control, but not eliminate them altogether. They are not a very good food for puffers because of their exceptionally hard shells (loaches suck the snails out, puffers crush the shells) which can actually cause damage to the puffer when he tries to crush it.
MTS should not be kept in a tank with an undergravel filter in place, as they cannot get back out of the substrate with the filter pulling down. They are great for planted tanks, or any other tank where the substrate getting anaerobic is a concern. It can be a shock to wake up during the night and see a hundred or more of these guys up at the waterline, but even in high numbers they are unlikely to do any damage to the tank. You can pick them out and crush them and feed them to the fish as you see them if the population is getting out of control.