I had 4 in a 160l and that seemed like a good amount - they hung around together most of the time. Never saw any agression. (EDIT - I have heard that keeping them singly can lead to agression.)
I kept them for about 4 years and they were voracious alage eaters throughout. Just my experience, I did feed sparingly.
They also like to chew on soft mosses as well. They don't finish them off, but they leave them looking stringy\tatty.
Hope that helps.