I would have recommended snails, but the fact that you have african cichlids in a 20 gallon tank kinda freaks me...
how many do you have? I hope you realize that the 20 gallon tank can in no way be a permanent home for them, since the amount of Africans you need to reduce aggression issues is way more than can fit in that tank when the reach maximum size.
I am reminded of a friend of mine who purchased 10 1 inch "mixed african cichlids" from walmart for his 10 gallon tank. they proceeded to kill each other as they grew until he only had 1 left, who I traded for a Buttikoferi for my 75 gallon I had at the time.