How can I stop this?

Or you could get an automated system in your tent linked to fans too. Expensive option but that what they are for. Like AC infinity among many

People have said it's lights on and off. But if I've read it right it's goiNg up and down within lights on and off? I have the same happening and it's all due to my central heating. I'm in the UK. If it's in a room with a radiator that comes on and off with a thermostat (in another room I presume) I'd turn it off at the radiator. Or leave heating on. Heating downstairs say would still affect it if your grow is upstairs.

Like someone else said if that's when lights are out then no need to worry.

26 is fine and you should have a dip in tempertures when the plants are sleeping. I would advice looking into larger extraction units :) if you pump in more cooler air this will reduce tempertures if you need to :) other than that humidifiers can cool down a tent and extra air movement fans can help too. Best of luck buddy

I'll assume it's happening when your lights go out. Personally I wouldn't worry about it, temp swings like that are totally normal. If you feel you absolutely HAVE to level it out, add a space heater that has an internal thermostat that's set to kick on when you're below your target temp.