After looking at the prices involved in purchasing 1000gal + storage systems i am thinking that i will just build a foam insulated concrete unit. They have a number of waterproof sealers available for the inside and it should be cheap and pretty easy. Just forms, foam, re bar and concrete. Plus the crete should add to the thermal mass. I have experience with concrete building forms, laying re bar and pouring on bridges plus smaller home based projects. Has anyone have info on this?