Where did you read that from lol.
Anyway look in my gallery, that room was at 85 degree throughout the grow with 2 dwc, and not only that the 2 dwc i had 1 264pump in there each to circulate the water(so that pump puts on more heat). So obviously you feel victim to the water chiller company that probably put it out there that your res temp has to be 70- degree. And i do agree with you that the water in the res will eventually be the same temp as the ambient temperature, but when its nighttime the water cools and when the lights turn on and the room heats up the water temp will take awhile till it get to that temp this is because plastic isn't a heat conductor.
70degree? i think fdd would call this BS!