Humidity/Temp control

Hey guys, I'm still kinda newbing it up here so I'm looking for advice. I have been growing in a 3'x3'x6' tent in a finished basement. I'm running a VIPARSPECTRA Reflector-Series 600W LED Grow Light, 2 desk fans for circulation and an exhaust fan, running on lowest setting. Im growing 3 plants in soil. I was running 24 light, exhaust fan turns off with light to maintain temp. I just corrected my light cycle to 18/6 because my plants have been stressed and humidity has remained low ~35%. Since changing to 18/6 I find my humidity climbing to 85% because the exhaust is off. If I turn the fan on, I lose my temp's consistency, tending to go lower during dark period.

I guess what I'm getting at is, how do I maintain a constant temp/humidity in my tent. The basement is already being kept around 65° and if I raise that during the dark period I'm going to eat away at my oil supply. I'm hoping for a reasonably cheap fix as I just seem to keep throwing money at my tent and feel like it's not making things better just more complicated.

Thanks in advance!

I use a small Honeywell tabletop heater pointed in at the lower intake hood. It can be put on a timer to turn on during cooler dark periods. It's cheap, uses 170w on low setting. You could even set it inside your tent, but I like to keep all electrical devices outside my tent.
https://www.amazon.com/gp/product/B00I4UVGHO/ref=ask_ql_qh_dp_hza