I lost a 6 month old Zebra Danio today following a routine 25% weekly water change. He shared a 20-gal tank with two other Danios and six tetras (Rosie and Red Phantom). The whole gang seemed fine before the water change, but all were acting panicked right after. My first thought was to look at the water temp, which it turns out had risen about six degrees. I keep the tank at 76, and my thermometer was showing 82. A few minutes later the Danio was dead, and one of the tetras was lodged at the base of an Anacharis, breathing but not moving. Now the tetra is swimming again, but he is still weak.
I also tested Ammonia, Nitrites, and Nitrates after discovering the problem. Zero ammonia, zero nitrite, 20ppm Nitrate. That's typical for my tank. Also tested pH. Read 7.2, which is a bit below typical, which is 7.4.
So, could a six degree rise cause such a sudden trauma? Nearly instant death? Clearly, the water I added must have been considerably warmer than 82, if it caused the blended temperature to rise to that level. But the water I added felt cool to the touch, with a slight stream of warmer water mixing in. Certainly not hot. I fill the tank from my tap (separate hot and cold valves mixing in one faucet), testing and adjusting the temperature beforehand. It's not an exact method, and sometimes the pressure on one of the two taps will drop after I start filling, changing the blended temp. That may have happened this time; the only evidence I have is the temp rise.