72 to 82 is the goldylocks zone. Lower than 70 and it can slow down your growth (but can cause purple color which is cool). Higher than 82 and you'll have to run CO2 (with higher levels of CO2, you can run higher temps). But, anything higher than the 80s and you'll be a watering fool. I try to keep mine between 74 and 78 degrees and they love it. When the lights go off, the temps drop around 10 degrees .... which is fine.