Heater Keeps Gong Up And Down !

Hi
Ok this is the first winter I have had my tropical fish . I'm noticing the temperature is going up and down . From 1-2 degrees !
The tank is quite near the radiator so this could be affecting it ?
Should I just leave it on the setting or keep adjusting it ? It was too hot for my liking earlier so I took it down a bit .
Ally

It is normal for the water temperature to vary by a degree or two; it's the nature of thermostatically controlled devices. They have two switch temperatures - one which says the water is too cold so switch on and the other which says the water is warm enough so switch off. In some heaters, these two switch points are a couple of degrees apart.

However, if the tank is near another heat source like a radiator, the water will get warmer than the heater is set for. And if the air in the room itself is very warm, as during a heatwave, that will also make the tank warmer than the heater setting. Heatwaves can't be altered by us but having a tank very close to a radiator can. If you think yours is too close I would give serious thought to moving it.
If the cause of warmer than usual water is outside the tank (radiator, heatwave) adjusting the heater setting will have no impact on the tank temperature. It won't switch on at all and the external heat source will continue making the water warm.

Bettas do actually need warmer water than most fish. Somewhere in the region of 27 dge C is perfect for them.

It's because the bacterium that causes finrot, columnaris, multiplies faster at higher temps, so the disease can go from bad to worse very quickly. It is usually suggested to keep the temp lower so the med has chance to kill it before it multiplies to make even more bugs.

When you read to turn the temp up when adding med, was that for finrot med? Because with other diseases you do turn the temp up, particularly with whitespot to get the parasite through its 3 stage life cycle faster.