Made major mistake last nite. Left new tank heater running since it appeared to be keeping temp stable at around 74F. This morning water was too hot. Didn't take time to measure but likely in the 120F range. Immediately started pouring cold tap water in tank. One fish died, others survived near cooler water inlet from NFT system. 

Two huge mistakes on my part. The heater is suspended in the top volume of water, about 5-8 inches below the surface. The temp probe was temporarily siting…