Cory catfish do npt eat algae. They are more omnivores with a lean towards protein. They lovee shrimp pellets, complete tablets and often will take to algae tablets, though I know of no cories that actually eat alage on plants. They can help keep alage low, especially on rocks, by moving things about. But I am sorry to say that if you bought them to keep your plants clean then that was not a wise purchase. Your fish store doesn't seem to know much about fish.
If you plan on keeping them then buy some sinking pellets for them. for 2 cories, 1 pellet a day will be more than enough. These fish also are much more active and fun to watch in bigger groups.
