My opinion but although I think Cleaner Clams are pretty cool, they don't do anything to clean the sand.
Do you know if the Nassarius you have are actually of the Nassarius Genus? If you post a picture of one I can tell. I ask for two reasons, Nassarius sp. are not detrivores per se, they only eat dead animal flesh. They will not eat fish poop, dead algae, or anything else, just dead animal flesh. There is a mud snail that is not really tropical that is commonly sold as Nassarius. These snails will eat algae, dead animal flesh, as well as live animal flesh. I don't recommend the mud snails for both reasons that they are not tropical and they are predators. If yours are Nassarius then you have way too many, two or three should be sufficient to clean up the corpses of things that have died.
Cerith Snails, Bristleworms, sifting Sea Cucumbers, Serpent Starfish, Cirratulid Worms, Peanut Worms, are among the most helpful in keeping a sand bed clean as is a good waterflow to keep the detritus suspended. Bristleworms are probably the most helpful. If your sand bed is large enough then a Fighting Conch can also be a good addition.
Within the sand bed there are many type of pods and microfauna that are needed to keep the sand bed healthy. So there are two parts, keeping the sand bed healthy as a good environment for the sand bed cleaners to live in.