I've had a 70 gallon cichlid tank for about 5 years. Occasionally I've lost fish do to my time constraints. Now that I'm retired, I can more closely monitor conditions. Until last week, everything was perfect. I added three fish (usually I add one at a time). Possibly overfed the first night. Nitrate levels went red, and Nitrite went up. Ph went down. Ammonia was zero. I've changed out probably 50% of the water and made sure the filter was clean. In the past two days, I've lost my big Jewel and both Pictus Cats. The cats stayed near the surface, while the Jewel remained at the bottom. After removing the Jewel, I noticed a slight slime coating. The newer fish are acting normally, but the other four are lethargic and hanging close to the surface in a corner. All but the Neon Blue seem to be eating just fine. The only other fish is a 12-year-old Plecostimus. Now signs of aggression such as fin damage (the Jewel was the only aggressor). Water temperature is a constant mid-70s. I have tried to raise the pH using limestone and ATI pH Up, with minimal results. Same with a detoxifier. I hesitate to add more of the chemicals, as I have already done the maximum according to the instructions. Current levels: High pH - 7.6; Nitrite - 0.25ppm; Nitrate - 10.0; Ammonia - 0.0. All of the cichlids are Malawi. I've had three for 9 months, the rest are more recent. I'd really like to save what I have left. Suggestions? More info needed?