Getting a little testy here aren't you? I was simply asking a question. I have done quite a bit of research on this particular topic, and your statement that the bottom of Lake Malawi is sand is incorrect. In point of fact, two different substrates are to be found in Lake Malawi, sand and rock rubble.
The underwater photic environment of Cape Maclear, Lake Malawi: comparison between rock- and sand-bottom habitats and implications for cichlid fish vision
According to AD Koning's book Cichlids and all the Other Fishes of Lake Malawi, while both substrates exist in the lake the Mbuna cichlids are tied to the areas of rock rubble rather than the areas of sand.
I apologize If I've diverged your thread, that was not my intention, based on your statements, I was simply continuing my research. Your statement about examining the Lake itself made me curious as to whether or not you had been there, as your "observations" were different than those who have.