He was talking about his water temperature, so 75F is way too high. (Because of low levels of dissolved oxygen and pathogens such as pythium's (root rot/slime's) preference for that temperature.)
From Just4Growers on ideal DWC water temperatures: "62-68°F (17-20°C). Above 72°F (21°C) and dissolved oxygen (DO) dips too low. Below 60°F (16°C) and plants tend to slow their metabolism as they "think" the season is changing. Some growers use this to their advantage and reduce nutrient temperature towards the end of the flowering stage to aid in ripening."