They eat the veggies. Veggies in the tank don't encourage algae growth unless they're left so long that they decompose and provide too many nutrients in the water column.
I have a flying fox in my tank, and he's a pretty good resident. He chased my tetras as a juvenile, but since he's hit 4 inches, he mostly just keeps busy cleaning leaves. Along with everything else in my tank, he did eat the mayacan I tried, but otherwise he doesn't cause problems. Loves the rind of the zucchini--he and the rainbow shark fight over the rinds.