Good question.Many fishkeepers forget about our friends at the bottom of the tank and think they can scavange and suck the rocks and glass to "scrape" off algae. This is simply not the truth. Algae eaters such as Plecs enjoy a change of diet like us all. Normally the multi-purpose tropical aquarium flakes does contain veggie matter, but to give your algae eaters a diet for themselves, I would personally recommend, Hikari's Algae Wafers and/or Hikari Tubilina. (Please note that I have no connection with the manufacturer and that my recommendation is entirely my own personal opinion)
Hope this helps.