Bit of a tricky one, at first I thought it was Gnathochromis pfefferi but I suspect it is actually a large old specimen of Ctenochromis horei thats lost some of its colour
Another possibility is Lobochilotes labiatus. This species develops characteristically enlarged lips in the wild, but I think that this may not happen as much in the aquarium and this may also be a young specimen as it's hard to tell how big it is.
@gentle lemur weirdly enough I was thinking that earlier and having had another look I think you might be right there. I know the spectacular lips found on wild caught adults of Placidochromis milomo do not develop on specimens raised in captivity so could well be the same for L. labiatus though can't find any evidence for this (though not surprising as this is a much more rarely kept fish)
