I agree with dogofwar, I'd go with a dwarf species like curviceps or apistos or blue rams. Even the midsized cichlids can have troubles in a 20 long, especially when the male is ready to breed and the female isn't. Unless you want to have a divider handy, I've lost females in 20 longs with convicts and other similiar sized fish. Now I only keep 3" or smaller fish in 20 longs, unless they are super mild. In fact, there are some dwarves I still wouldn't keep in a 20 long due to their aggressive nature.