I have 3 "fighting" conchs in my 120. They help a lot. Good water movement is always good too. Also, good water quality (ph, alk... And low nutrients.)
+1 to all the above. I had a sand sifting star, 2x conchs, a bunch of nassarius snails, a mp40 creating an undertow, and a tiger tail cucumber in my 120. Mine was pretty clean.