If the temperature of your nutrient solution is too cold, then the growth of your plants will slow down. If the solution gets too warm, it will encourage root diseases like pythium. Plus, the warmer water gets, the less oxygen it can hold dissolved in it. The ideal temperature of nutrient solution is between about 18C - 20C (65F - 68F). During winter, you might need to put a water heater into your hydroponic tank to keep the temps in the right range. In summer, the reservoir temperature can rocket (particularly with hot grow lamps on in the room) so a water chiller will help keep the roots healthy and happy with plenty of dissolved oxygen.
