Digital thermometers and heaters cause a lot of stress for some fish keepers. Before they came along a quick glance at the old analogue thermometer was enough to know that your heater was working and temp was fairly stable. Now we can fret over small fluctuations that were not evident in the past. In nature water temp rises and falls with the seasons, the time of day and the weather. Fish don't start dropping dead. A reasonably stable temp within a band of a few degrees is fine for your fish.
I have three means of reading the water temp, they vary by a couple of degrees from each other, I just take an average to check the heater is functioning correctly. More importantly is to use similar temp water for your water changes so temp doesn't quickly rise or fall.
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