Noble's Pureau water cask has a mylar bladder in it. After drinking the water (rather than using it to make the Dektol which was already made up), I inflated the bladder by blowing into it like a balloon and waited a few days to see if it deflated under pressure . Thankfully it faithfully held its girth ! So then I pryed off the plastic tap thingy, transferred the recently mixed Dektol stock solution into the bladder, expelled the air, re-capped it, reshaped its cardboard box and stuffed it back inside then taped up the box. I expect little colour change of the Dektol after 9 months ! Previously my PET bottles with gaping holes under the lids let lots of fresh oxygen into the sol'n, oxidising the Dektol in a matter of a few months.