How often?

[Alleycat]

MTS = Malaysian Trumpet Snails.

They're great for all tanks, to help eliminate any gas pockets in your sand or gravel. They will also help with some clean up, but they really can take over a tank quickly if you're not careful.
I guess my fish are used to me cleaning their tank with a hose as they are not stressed and re-produce more than I can keep up with. Like moonie, I don't go close to a shell bed with fry until the fry have been removed.
Plecos like the one pictured, are one of the largest waste producers next to the common goldfish.

[fish head]

MTS are Malaysian Trumpet Snails and they are a pest to a lot of folks.......can take over a tank!  They come free on "dirty" aquarium plants that you buy at a fish store, or in the gravel/sand from someone's tank who's infested.  But they do a great job of going thru the sand, like earthworms in a garden, and they are a part of the web of life.  They also come out at night and eat algae.