Never had much luck with nassarius snails being of help in the tank either. Ime, they actually need to be fed... so do not count as clean-up crew in my opinion. Over the past couple of years, I have had a total of 5 of these snails (not all at the same time) and the only ones that lasted very long were those that erupted from the sand during feedings, at which time, I'd feed them.
I've heard very good things about the tiger conch, a dwarf conch relative to the big guys found in the tropics. They are omnivores reported to eat all kinds of things from detritus to leftover foods to algae growing low on the rockscape. The downside for nano aquarists is their size. I have a 20g long, went to buy a tiger conch one day, took one look at him and decided that I didn't want a 3-inch snail in my tank. I do know of some nano keepers that love these guys, so it is a personal choice.
In case you're interested in what keeps the sand clean in our little reef tank... well, the sandbed is not too deep and we are pretty strict with weekly water changes. I do not want a thorough clean of all the sand every week though and that's mostly because there are beneficial organisms in the sand and I don't want want to put too much of a dent in the population all at once. So, 50% gets cleaned at one water change and the other 50% at the next.