ive kept africans for quite a few years.. and love demansoni's. it looks like bloat to me, i have had quite some experience with bloat and africans.. its what led me to get away from africans altogether.
they need spirulina. and the cichlid formula you are feeding may have too much protein for africans. they make food specifically for africans (NLS brand). also, add veggies into their diet. i used to feed them frozen slices of zucchini, squash, cucumber, cauliflower.. etc. just rubberband it to a rock and sink it.
as far as bloat goes, it is hard to treat and i never could save my fish even with epson salt added to the tank and metronidazole or flagyl. bloat is thought to be caused by a protein rich diet, misuse of aquarium salt with africans - i stopped using salt altogether because i felt it was secondary to the protein problem. and stress also can trigger bloat.
seeing you lost a female, a alpha male i would say that stress in the tank may have triggered the bloat. i would take out the bloated ones, treat them for bloat and not feed them a few days.. and i would actually get more africans for the tank-since overcrowding actually helps disperse aggression with africans..
just my .02 cents-ive dealt with bloat and africans alot, but others on here may have better advice.
and everyone else is right about the nitrates in the tank-they should be there if the tank is cycled regardless.. sometimes its hard to tell with those chemical test kits even in direct sunlight.. lol. best of luck to you and your africans!
