the ones you saw in the fish store are probably false zebra plecos or columbian zebra plecos (L-046). a true zebra pleco (L-46) cost hundreds of dollars.
as with any algae eaters, you have to supplement their diet with prepared foods such as sinking wafers, flakes, fresh veggies and etc. please don't purchase a fish because they will help you clean the algae. use some elbow grease instead!
im also curious to know if the pleco in your 20G is a common one or another variety. many pleco's aren't small enough to live in a 20G.