I know of two people that 1000 gallons stacked. It works well for them. You would need a minimum of a 80 kw boiler.
henfruit, how come such a large boiler is required?
Spray your tanks with 4 to 6" of closed cell foam first then cover with a foot or so of cellulose or fibreglass.
Thanks more like it... This sounds good. Is the spray foam good with the heat? Does it have to be rated for higher temp?