Otocinclus are varatious algae eaters, so you wont need many in a 55 gallon tank, maybe 4 tops. While this will help you keep it under control, make sure you sort out the source of the algae issue though.

Once your aglae is gone, you might want to feed them extra tabs on the bottom, a fat Oto is a happy Oto Any fish shops will know what you want if you say you want Oto's. Also consider shrimp as well, shrimp are natures gardeners and amazing tank janitors, plus theyre amazingly interesting to watch and breed like rabbits. Neither will eat your plants, although Oto's might nibble on soft foilage if theres nothing else to eat.

Oto's are very good on broad leafed plants, vallis and wotnot, they do struggle though with narrower grass like leaves as they cant get their sucker mouths around them to scrub them clean quite as efficiently, thats where my shrimp take over.

I found this site really good regarding Oto's. Be careful how you acclimatise them to your tank, take it real slow, theyre notoriously hard to get settled in, but once they are, are very hardy.

www.otocinclus.com - Home of the Dwarf Suckermouth Catfishes