Successful hydroponics is all about control. You control the exact nutrients available to plants, the pH of the nutrient solution, even how much sunlight (either natural or artificial) the plants receive. The temperature of the nutrient solution you use determines how much dissolved oxygen is available, as well as which pathogens can survive to attack your plants. Keep the solution at around 68F to optimize dissolved oxygen while minimizing pathogens.
Use a Commercial Water Chiller
Hydroponics suppliers sell a variety of water chillers designed to regulate the temperature of a nutrient tank. Most models have a built-in thermostat so they can turn on and off as needed to maintain optimal temperature in the tank.
Commercial chillers tend to be expensive, ranging from $300 to several thousand for large-scale hydro-farm models. They are also very reliable and efficient. Most are combination heaters and chillers, so one unit handles all temperature issues. A low-end commercial chiller can lower the temperature in a 100-gallon reservoir by about 10 degrees.
Store the Tank Remotely
The ambient temperature in an enclosed greenhouse tends to get hot quickly, especially if you are using grow lights to supplement natural light. Move the tank to a temperature-controlled room, and the water will adjust to that ambient temperature. This is a good solution if you have a greenhouse built onto the side of your house and you keep the ambient temperature of your house around 68F.
You will need to drill a hole in the wall between the house and the greenhouse to run the intake and drainage tubes from your garden to the reservoir. Be sure to seal this hole well after you run the tubing.
Insulation and Ice
If you cannot relocate the tank and the cost of a commercial chiller is beyond your budget, you can still cool off the nutrient solution.
First, insulate the tank. The easiest way to do this is by re-purposing a large camping cooler as a new nutrient tank. Coolers are already well insulated, and most come with a drain spout for water changes. If this solution proves impractical, wrap your existing tank in several layers of insulating material.
Insulating the tank will protect it from ambient heat, but you also need to cool down the nutrient solution. Fill 2 to 4 gallon containers (old milk jugs work well) three-quarters full with water. Freeze them overnight, then put them in the insulated nutrient reservoir. Keep another set of containers in the freezer to replace the first set as they melt. Return the first set to the freezer, so you always have a supply of ice packs.
Do not use freezer packs for this purpose--they tend to crack and leak over time.