The Macrotocinclus affinis, formallly Otocinclus affinis, commonly called Oto or Oto Cat is a great algae eater that stays small. They rarely get over 2 inches.
When I left a friend in charge of a 29gallon heavily planted tank, he wanted to see the fish longer than the light timer allowed, so frequently turned the light back on. After two weeks, the entire tank was covered in brown algae.
A shoal of 5 Otos cleaned the tank in 4 days. I have to feed them algae waffers and fresh veggies (they love zuchini and peas) since I do not have enough algae to keep them full.