i wanted to know this too and did some searches, most people indicated raphael catfish so i got one, i never see him ever and i don't notice my substrate any cleaner so ... other recommendations were pictus catfish and synodontis eupterus. i think clown loaches would work too and also very cool fish.
blue channels, bullheads, .. cory cats if they are big enuff, the first 2 dont really keep the substrait clear but they are sweet bottom feeders, snaild are ur best bet... huge apple snails.. or trap door.. or mystery..
Synodontis or Pimelodella catfish are great...I wouldn't get any bullheads or blue channels, they get way too big...especially the blue channel cats, they get over 2 or 3 feet in length.
blue channle wont get 3 ft in a tank.. i rairly cats 3ft's in the river near my house .. maybe once or twice, but they will never get 3ft in a tank. and bull heads will max out around 8 inches in a tank, plus both of those fish can take a real beating
110... I'm leaning towards clown loaches at the moment, they seem like the obvious answer. I have a syno in my other tank and doesnt seem to eat off the bottom, he seems to pick fights more than eat. I was thinking about corydoras too, but like you said.." If they are big enough" which they are hard to find big. I set up a tetra tank for my mom and she has about 10 corys in the bottom and I rarely have to clean the tank. Maybe a school of clown loaches... would add some more color to my tank as well.