I have a 79 gallon freshwater aquarium that has been operational with fish for approximately one year. I had a large pleco in the tank for approximately the first four months. This pleco came from my 39 gallon tank where he lived comfortably for about two years. This pleco shared the tank with neon tetras, cardinal tetras, rummy nose tetras and corey cats. Suddenly my pleco developed what appeared to be lesions on his tail area. Shortly thereafter he appeared to have a major nervous system problem and this pleco did die. All six corey cats died one by one. All tetras remained healthy. I've since tried to re-introduce plecos and coreys into the tank several times and always with the same results. Within two weeks they are dead. The same tetras live on with virtually no loss of life. I have algae in the tank. I change the water once per month (approx 80% change). I keep the temp a steady 78 degrees. The tetras are quite healthy. Why can't I get the algae eaters to live? Suggestions would be greatly appreciated.