Long spined urchin

Depending on how much algae and waste you have in your tank will determine if you need a larger cuc...but if you don't have much, then the urchin will do a good job, and if you get two, they won't fight...I've never heard of them fighting each other.
i have 2 in my tank and there are no issues, my tank is a 58gal. and i could probably get away with a couple more, i am stocked pretty heavy on fish for my size tank though so......