The weird thing is you may be looking at the Nile tilapia which fits your description. They are considered food fish here in Asia so it is extremely rare that you will see them in home aquariums. Treated as food, these fish are not given the inch per gallon rule though they really thrive in denser conditions and are very hardy. There are many decorative and aesthetically pleasing species of the Tilapia though technically not belonging to the genus.