Hmm, mixing nutrients in warm water isn't something I've heard of before...

I can't see any real advantage to it either unless their dry nutrients in which case that would make them more soluble...
The only thing I can think of is for soil growers they wouldn't want to shock the rootsystem with freezing cold water... Any quick temperature change of more than 10 degrees will shock them for sure.
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id be willing to bet you could build achiller or a probe style chiller even from a pos window a/c unit and a some work
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There have been plenty of aquarium dudes who have done this (search google) and their general conclusion is its worth the $ to buy a prefab unit.