I'd keep my heater in, set to what degree I wanted and then I would do the following:

I would buy a mountable fan, and have it blowing under the hood, and/or I would invest in a chiller. It's warming up for summer, and if you are already having overheating issues, it could get really bad in one day of heat.

Chillers for your size aquarium run around $100, and sometimes on Ebay, you can get one that is less money.

You run it, controlled, at your desired cool, and the heater will still prevent a drop.

The fan may just do the job for you, but if not, please consider the chiller.

I can recommend a little Iceprobe, which is sufficient for a nano tank, and I see them all the time, on Ebay, for $80 to $100.