I would suggest a plecostomus; the Blue-chin Xenocara is an excellent algae-eater, and only gets up to five inches in length. A more common--and much bigger--variety is the regular plecostomus, Hypostomus plecostomus which get up to two feet in length, but may be much easier to find and probably much cheaper. I would go with the plecostomus because, unlike the friendly little corydoras catfish, these can be semi-aggressive as well (you only need one!) and, from what I've observed, they can even be kept with fish such as the Oscar and go untouched. For what reason, I'm not sure, but all the fish I know of appear to leave these alone. I am not entirely certain, however, so perhaps you may hear some better advice from a more knowledgeable member of this forum than I.
