My comfort zone ends at 72. Few days above that and my rez won't smell like fresh vegetables anymore. Not that my plants mind a lot, but I'm thinking of eating my roots next harvest
Try to keep it below 70, your plants will thank you for it. Less chance of root probs, better DO. Watch closely and they start cheering around 66.5. 75f rez temp is kind of like 91f environment temp. Won't kill your plants... won't make 'm happy either. Best thing about hydro is control, and that includes rez temp.