Water chiller would be best and I'd say 60-75 would be the very high and low but assuming ur res is remote u can try a frozen bottle of water but if ur dwc I wouldn't reccomend introducing any ice to the roots. OR I saw the other day a guy ran a hose from the pump in his res THROUGH his freezer and back to his res keeps it running constantly and says his temps are pretty steady and his only problem is sometimes it gets too cool. But not everyone has a freezer layin around they can stab holes in