In most cases, nutrient solution should be provided at or near the ambient temperature in the growing area, or whatever is typically supplied to the plants. Water that is exceptionally cool can shock the plants, and will often trigger a growth slowdown or even dormancy. Water that is too warm can limit the uptake of nutrients and stress the plants. In addition, warm water cannot hold as much dissoved oxygen, which then creates another damaging situation for your crop.
The nutrient solution in your reservoir should be kept in the range of 65 to 70 degrees Fahrenheit. This temperature promotes optimal nutrient uptake while limiting the negative effects of water that is too warm or too cold. Be strategic about the placement of your nutrient reservoirs, then. If you're growing in cold weather, avoid storing the reservoirs near the edge of your enclosure, where heat can be lost through the walls of your tank. Similarly, avoid placing your reservoir near heaters. If your enclosure's temperature dips overnight below the minimum 60 degrees overnight, consider adding an aquarium heater or other gentle heat source to your reservoir, or even wrap it in insulating fabric, similar to what you use on water heaters.
