55 gallon african cichlid tank

Hi,
my wife and I have a 55 gallon african Cichlid tank we have a total of 10
cichlids and we were wondering what would cause a fish to discolor our
labidochromis caerulues (electric yellow) is now a brown color and we are
not sure why, he doesnt looked stressed we have had our water check on a
regular basis to see if maybe it was a ammonia problem the water checked out
fine our PH lvls are good and we were just wondering what it could be, we
have 2 Pengiun 350's as our filters and a bunch of real rocks, any ideas
would be great as the only other people we have to ask is around here is the
local petsmart (not saying anything bad ) but they are not the best of
advise for cichlids we went ahead and put ammo lock in there just to be safe
than sorry and we did a 15-20% water change as well


Thanks
Jason