The ideal temperature of nutrient solution is somewhere between 18°C and 20°C (65°F - 68°F). Nutrient temperatures that drop too low will stress out your plants, slowing growth down to a crawl. During the colder winter months, a tank heater will prevent these drops and keep things running sweetly. When nutrient temperatures creep above the danger-zone of 21°C, levels of dissolved oxygen start to drop and the door is opened up to various root diseases, including Pythium. If you're having difficulty controlling high nutrient temperatures, a water chiller will do the work for you. It will keep roots healthy and happy, with plenty of dissolved oxygen.