Temperature Variation

That's a great question!

I have the same thing. I have a digital thermometer and the standard supplied 200W heater.

I've noticed that in the morning after the lights have been off for 12 hours or so I'm seeing the temps down by approx 1.5 degrees C compared to when the lights have been on for a significant period.

I'm leaning towards thinking that in nature the temperature would fluctuate naturally but only over fairly long periods, as is what seems to happen in my tank.

I'm guessing that if the temperature were to change by a degree or two over the course of a couple of minutes then the fish would be shocked by this but with gradual changes they're OK.

Anyway, I'm thinking aloud.

I'm looking forward to an expert's opinion.

All sorts of things contribute to the temp of the tank.
The heater runs on a thermostat and switches on and off to maintain temperature by a feedback mechanism. It cannot maintain the temp to within a fraction of a degree and probably only to within a couple of degrees or the element would be switching on and off every few seconds.
The water will be warmer when the light is on as that warms the air above the water. The heater will be on less often as the lights warm the water. Then when the lights go off, the temp will drop until the heater turns back on.
The weather also affects the temp. Not so much in winter as the heater keeps the water warm but in summer the temp can rise above the setting on the heater. In a heatwave, we do save on electricity but can have problems if the water gets too warm.

In nature, other things affect the water temp.
Night time is cooler than daytime.
Clouds covering the sun will cause the temp to drop.
Melt water from mountains in spring can drop the temp of a river by several degrees - something used by cory breeders who do water changes with cold water to simulate this and stimulate cory breeding.



As Simon says, it is only a problem if the temp varies a lot over a very short period. A couple of degrees over several hours is not a problem.

A lot of people with large tanks use a hose pipe to empty and refill, and use cold tap water when refilling. As long as this is added slowly it is fine. I don't heat my bucket water to the same as the tank, but I do add boiling water from a kettle to warm it a bit as I add it quickly.
If you have a combi boiler, you can use hot tapwater; if you have the type of set up with a hot water cylinder and a header tank in the attic, use a kettle. This isn't because of copper dissolving from the hot tank but because of things that could be in the header tank.