I've kept mud snails in various aquariums for years. These snails are a lowly marvel of nature. Black and nondescript, they can survive out of water for long stretches, can handle water as warm as a bath tub, and even quick changes in salinity. They eat everything, including scum, but are especially attracted to chunks of fish, and it's amazing how fast their little sniffer tubes will shoot up and start sniffing if you drop some fish nearby. In no time at all the dead fish will be just covered with the snails. After being in my reef tank for a while they'll be coated with purple coralline algae and be a lot nicer to look at.
The worst part about these snails is they carry swimmer's itch, which I discovered one day after cleaning my aquarium in which I had recently stocked lots of mud snails. I soon had big round horribly itchy welts all up my arm where microscopic flatworms had mistaken me for waterfowl, crawled under my skin, and promptly died. The flatworm's first host and carrier is the mud snail.