I have had many people recommend burying your reservoir and or creating some form of "geothermal cooling" by using buried coils with a
pump.
I myself found a used aquarium chiller from someone breaking down an aquarium and bought it for 150 bucks. I can manually control my 55 gallon reservoir to seemingly any temperature from 55 degrees upward. I keep my tomato
nutrient res. at 67 degrees.
It may be more expensive than other methods but the reliability and programmable nature of the chiller has been great for me. I am a new grower however, and others surely have better/cheaper alternatives.