I do not believe synodontis eat algae, even though they are bottom-dwellers. Actually, I think they are carnivorous. I hope you took your pleco out if he was getting beat up. I hate hearing about plecos getting beat up in African tanks! (Is this an African tank?) Perhaps you could look into why you have algae. I have none in my tank, but I only put the lights on for viewing and it's not near a window so it doesn't get direct sunlight. The fish don't need the lights, so unless you have plants, perhaps you could turn them off? Just a thought . . .