OG PK / Northern Lights Flower fr. Clone

This week we have noticed a lot of changes! The Northern Lights is capped off at about 28". She's looking very frosty and her gorgeous red hairs have started to pop. The OGPK has capped off at about 25". She is also very frosty and is getting nice and thick! We have started using Advanced Nutrients Overdrive for weeks 5 & 6 and will do our last 4 waters as a flush in our final weeks. We are looking at an 8 week flower. Our tent dropped just below 20 degrees with lights off for the first time on day 35. We had the temp with the lights on up to 24.5 degrees on day 31 but cannot seem to keep it consistent. I feel like the basement space in the colder days we've been having has something to do with that. Any suggestions on regulating temperature?