I have a fully cycled new 75gal tank (about two months). Parameters are perfect and have been double checked on different types of test kits. pH is stable too. Filters- aquaclear 500 and hydor pro rated for 90-120, charcoal, bio balls, ceramics, and floss. I have lost over ten fish in the last month. At first I thought they were dying from shipping stress and the cold, but now I have locally bought fish dying. When I move fish to my small 15 gal to rest they get better. The strange part is, is that there are congo tetras and pictus catfish that have been in the awhile that are perfectly healthy. The only thing in the tank is black caribsea sand and local rocks that vinegar tested safe. The fish act okay for 2-4 days and then go pale, act reclusive, and suddenly die. Thoughts? I started off stocking with acaras, geos, and firemouth. I switched to Africans after to try something local. I have tried salting the tanks according to instructions and adding a slime coat builder.