Why not transfer it and DI on demand ?
Set up my rainwater IBC yesterday and now half full with TDS of 006 - 007.now need to transfer this to van tank but need to put it thru DI Vessel first to make it 000any idea what pump I should use cos if i use my submersible pump I think it will be too powerful maybe summit like this ?http://www.machinemart.co.uk/shop/product/details/051011800