I picked up 6 rainbow cichlids (Herotilapia multispinosa) about a week ago from a LFS. They're sub-adults, about 2-2.5 inches. I have them in a heavily decorated 40g by themselves currently, with water parameters of 8.2 PH, 0 ppm ammonia, 0 ppm nitrites, 20 ppm nitrates.
I've never had rainbows...