corydoras catfish are terrific bottom-feeders, and readily available in all LFS and major petstore chains . . .
if your tank is big enough -- 55G or more -- you might also consider yo-yo loaches; these don't get anywhere near as big as clown loaches, and they have a lot of personality
in either case, try to get at least 3 or 5 of whichever one you go for; they do better when they are in small shoals . . . and, apparently, in odd numbers