Nerite snails are awesome algae eaters and have a great reputation for it on TPT.
SAE while great at eating pretty much any kind of algae, get aggressive when older and not usually suitable for smaller tanks.
Otos are great algae eaters once established, but can be hard to acclimate at first (which is why they are suggested for mature setups only). They love buddies also, and I think 3 would be fine in a 20 gallon setup.
Smaller plecos (bristlenose, bushynose, medusa, rubberlip, bulldog, etc.) usually reach about 4 to 5 inches and great at eating algae at first, but can become lazy over time. Depends on their individual personalities. They can also go after broad-leafed plants.
With any option, once the algae is gone you will need to supplement their diet with algae pellets or some kind of veggie. Good luck!
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Last edited by Ashnic05; 03-17-2013 at 12:32 AM.