Ottos won't churn the sand at all. They are basically
aufwuchs feeders, meaning they are adapted to living in rocky streams where they graze on green algae and the tiny animals within it. They will also take a variety of aquarium foods, but only relatively soft ones. Mine enjoy slices of courgette and cucumber, but will suck the juices out of a bloodworms, too, leaving the empty husks behind. Ottos are tricky for the first few days, but once settled in their relatively low maintenance. They can be annoying by sucking the mucous from larger fish though, so they are best used with small, fast fish like tetras. Not with anything slow, like a goby or cichlid.
Almost all bottom dwelling catfish will move sand about a bit, but
Corydoras and
Brochis are two nice fishes to think about. Loaches can be good too, but choose these with care as many are pretty mean tempered. Spiny eels, mormyrids, some puffers, and most gobies will also do some digging, with how much depending on their size.
Some snails don't really make a tank dirty any more than fish, and Malayan livebearers certainly don't. Unless you come down at night and watch them, you probably won't even know they're there. Most of the problems people have with snails are because snails are (a) hardy and (b) eat decaying plants and fish. So as soon as an aquarium starts going wrong and the plants and fish die, the snails thrive. Hence,
post hoc, ergo propter hoc, people blame the snails for the dead plants and fish. There are some snails that eat live plants, and there certainly are some (marine) snails that catch and eat fish, but generally snails benefit from our mistakes, they don't cause problems themselves. Population explosions of snails, for example, come about after consistent overfeeding: what the fish don't eat, the snails do.
Snails do produce slime, but I've never heard of a tank being made a mess from it. It is water soluble, so as soon as the snail has moved along a bit, the slime trail behind it dissolves. It isn't like having slugs in your garden!
Cheers,
Neale
I don't really want snails though as i've heard they make the tank slimy and dirty, i'd rather have plecos or shrimp or something. Still, nobody has answered my question, would they churn the sand regularly to keep it ok?