Hi,\n\nSo, my tanks are all mature and have been stable for quite some time. In the last 3 days I've lost 3 fish, one from each tank - my yellow-flanked fairy wrasse (big 5+" fish) that I'd had for 2 years, my leopard wrasse that I've had for 8+ months, and my firefish that I've had for a year and a half. The only common denominator across the 3 tanks is the top off water. Nitrates are a bit up but not to the point that it should affect my fish. Is this a coincidence or is something going on?