How many gallons is the tank? Is it heated? (it should be) Are there any other types of fish in there?
I personally am a fan of shrimp as cleaners. I have a hand full of cherry red and blue shrimp in my guppy tank. They do a marvelous job at keeping my floors and ornaments clean.
I also recommend loaches. I have a few khuli loaches, they're adorable little guys and pretty good cleaners. You don;t have to have them in groups, but you'll see them out and about, playing with one another more often if you do. Make sure you know what kind you are getting as some, like dojo loaches, can get pretty big.
You could also check out cory cats.
