Agree with Milingu on either the test results are wrong, or the tank is not cycled. You should always have some nitrates in a cycled tank.
I don't recommend all-male with mbuna in any sized tank, and all male anything in a 40G tank. When you have males and females, you want 1m:4f at minimum of every species. With some aggressive species like you have, 1m:7f. Having unidentified females in the tank tends to cause problems.
A 40G tank is usually 36" or 48" long, what are the dimensions? For kenyi, maingano, johannii, elongatus I would want 48x18 minimum. Auratus tend to do better in a 72" tank with 1m:7f and maybe 2 other aggressive species. Demasoni could be OK in a 40G, but only as a single species with 3m:9f.
And just to throw it out there, as I know you are convinced otherwise, aggression is often not witnessed by the fishkeeper. Where do the johannii hang out in the tank, are they ever lurking under the surface or near heaters or filter intakes? Before they die, what do the feces look like? Thick and food colored? Thin/thready and white/clear?