Rapid temp changes can kill fish and/or increase stress. That together with existing stressors can add up resulting in dead fish. In most cases however the temp changes need to be at the lower or upper lethal limits. That is usually not at 60-80 F temps. Rapid changes in the 95+ or below 45 range can easily kill common southern pond fish. A common DO turnover event in summer (thermocline reversal)is a cloudy day or two with a heavy T-storm (cold rain).