I sure hope those rocks and decor is not just sitting on the top of your substrate.... You do know those cichlids are diggers and will make your rocks fall if not planted firmly on the bottom of your tank.
my suggestion would be to add a couple of hundred pounds or so more rocks...but not big ones..about 10-12 inches diameter....settle everything into the bottom...then pile them all up..a lot of fish in the tank..they need lots and lots more places to hide...
black gravel may look good ; but i would think that the needs of the fish should come first...they need a PH of about 8.4 for malawi fish and a little higher for tangs and vics.
crushed coral or dolomite would take care of that......
but as i always say....your tank..your fish your money...do it how you want it....