We have a 55gallon plastic drum in the utillity room with a auto shut off that our RO/DI drips into and we use it for pretty much everything. Water changes, coffee, koolaid, Ice tea... As far as storing mixed water you really need to have a power head moving the water till you use it. But it shouldn't sit too long
RO/DI should be "0" on everything unless your filter inserts are bad then you would have nitrates. If they are there your test should work on fresh or salt. There is an artical in the newest Aquarium Fish mag this month that touches on this.
sarah