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 on the molded ledge of the RubberMaid Ag tank. As long as the heater and the probe are in the same general zone of water, temperatures remained stable. However, at some point -- likely when I closed the tank cover, the probe dropped to the bottom of the tank. The thermostat then sought to heat the water in that zone, while overheating the top layer of water. At least, that's my theory. Stupid mistake on my part. Should have done more extensive testing while I could monitor it every hour, which I did yesterday.
Remaining fish seem okay. Will need to rethink water heating set-up. Think I need to put heater element closer to bottom of tank or make sure the probe stays fixed in the same zone. All future experimentation will be done in the sump tank first.
Live and learn!
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