I don't know if the fact that I added 6 large pieces of Texas holey rock to my tank is actually the cause of what I am experiencing or not, or just a coincidence of what has been going on. I have a one hundred gallon African cichlid tank with Peacocks, Brichardis, and some assorted cats and a Plecostumus in it. It is a well established tank; its been running in this house for 13 years. I have never had a problem with any fish, but after I had added the rock, fish started getting sick. 2 out of the 3 spotted cats in the tank developed Velvet and died (I treated the tank for velvet and did all the required water changes with the medication and all was well for awhile), 1 angelicus cat died, and 2 older Peacocks died. The Peacocks and the cat just went behind the rocks and refused to eat and became lethargic. The Brichardis show no symptoms of any illness and are in the tank with babies and all are fine. I also have a plecostumus in the tank, and he is just fine, as well. It is truly a mystery! Yesterday I cleaned the tank, and now another of my peacocks is exhibiting the same symptoms. I don't have any idea what is causing this! any advice would be really appreciated!