I don't think that heating up of the water is really a problem as it takes an awful lot if heat to raise the temp of a reasonable sized tankful of water... As for just using your tank water on the garden? What a bloody waste! Here in the country it's bore water for the garden and the dunny and the rainwater (filtered) is for the washing and the drinking. I have 12,000 gallons all up and in 20 plus years here we have never come close to running out, dishwasher, auto washing machine and all. Mind you, with the current drought I could be in strife this year if I'm not careful.