Hi.
Put two new lab's into my malawi tank. They died overnight. Checked my water as all other fish were fine. It is a well established tank. I discovered a very high nitrate level of 140. Could this be waht killed them. I only do water changes once a month on the advice of my retailer. My tap water contains no nitrate so that is not the problem. I have done a 50% water change today and will do small one each day for a week to bring levels down. There is no other problem in the tank they were not attacked.