Kribs are nigerian but there are two types of "african cichlids" - those that live in the rift lakes, and those that do not (kribs and tilapia fall in the latter category). Even rift lake cichlids fall into subcategories depending upon which lake they live in. Lake malawi cichlids are more tolerant of water changes than lake tanganykia (sp?) ones.
The coral in your substrate will tend to buffer the water and make it harder over time. This is fine for central american cichlids (although if your local water isn't hard and alkaline if you wait more than a week between water changes and don't do large ones, the parameters of your tank water and tap water may be rather different). Kribs and SA cichlids may be a little less excited about the effects of the coral, so I'd take it out considering you have severums and G. B.
It is true that ich is ubiquitous, and that stress can lower a fishes immune system, but most cases of Ich IME have been from infected fish. With thousands of huge water changes under my belt, I can't tell you of a case of Ich brought on by them. If anything, netting the fish probably caused more stress through partial removal of the slime coat than a water change ever would. Keep in mind I didn't make this stuff up - way before internet forums were big I was an ACA member and talked to dudes like Paul Loiselle, Wayne Leibel, John Stankevitch and Tom Ganley (the latter two of which I even bought fish and tanks from).
