Although you asked about flake, I and all my Mbuna keepers find that you will not get the same growth rate by using flake...I started using NL Spectrum pelleted food and I have to admit its the best I have ever used, the growth,colour and overall health of the fish was clearly visible against some of the same batch of fish fed on s/flake.
Link below http://www.nlspectrum.co.uk/product_freshwater.php?p=freshwater