You can give them some veg if there isn't enough algae. One of mine is currently eating a slice of cucumber. They also like courgette, not sure what else. I have 3 in a 125 litre tank along with tetras and corys and they all get along great.
as i said over in the pleco section, i don't think adding them would be a good idea, they ideally need larger tanks therefore having stable water conditions IMO a 35l just is not big enough.
instead i would find out why you have algae, common cause is too many nutrients usually caused by over feeding
I don't think they need a massive tank. I have a really good aquatic manual book and in that it says 45cm tank would be all right as so for guppy's that I have and are very healthy. You may be right but I really want 3 or 2 of them. Also the staff at my LFS say a biube is fine. I have a very bright light. That could be the cause. Would live plants sort the problem out?
I am going to go back with the otos because other people have had them in the biube and they have been fine. I respect the other members opinion though. Thanks for the helpful info.