As Neil mentioned, water parameters enter into stocking and they also affect a fish's behaviour. Fish that are having to work harder, physiologically, because of parameters for which they are not suited will be under significant stress and this impacts their level of aggression (it can either increase or reduce a fish's inherent behaviour/aggression level), weakens the immune system, and other things.
Leaving that, there are other issues here. The Jewel cichlid will attain around 6 inches normally, though in the wild it can easily attain 8 inches. If the two are male, trouble will inevitably ensue; if they are a pair, spawning will likely follow and the other fish in the tank will not be well accepted. But then there is the Red Devil, a fish that should reach a foot in length, and requires a 50g tank on its own.
So yes, on more than one front, your tank is overstocked, which partly includes mis-stocked.
Coming back briefly to the water parameters, the rift lake cihclids must have moderately hard to hard water with a basic pH that should be at or above 8.0 if they are to be healthy.
And a last comment, picking up on the "now it seems everything is a peace,"...this is not a good omen. None of these fish are "peaceful," and as I alluded to above, fish that are forced into unnatural conditions, be it parameters, inappropriate tankmates, tank too small, or an improper environment (the aquascape), will be stressed and this is going to weaken them and shorten their lifespan, inevitably. Unnatural behaviours is a sign this has probably begun.
Byron.
