@dalebout123 I'm not positive that the metabisulphite solution can be stores in plastic. I try to minimize my exposure to plastics anyway, which is why I prefer bottled beer over plastic-lined cans. Not that I drink much at all these years with a wonky heart (alcohol was never big in my life, thankfully, even with 30 years in the Army. Not muh thang.). You can purchase replaceable stopper bottles of beer, like Grolsch, or others, the ones with the heavy wire loop and ceramic stopper. Get a large liter bottle with that stopper, sometimes at those same DIY wine/beer stores. Store the solution in that. Note that, with exposure and time, the rubber gasket WILL harden and crack, as I have learned. Some of that is natural deterioration, but some is exposure to the sulphite....so that's why I tend to avoid storage in plastic. It's all organic, after all.

Yes, rinse well....everything you make the bisulphite contact. This is mostly for comfort, not quite so important for exposure, or unnecessary exposure. Your nose and taste buds will let you know you should have rinsed. The residue is minimal and should not harm you at all, but yes, rinse, please.

I have noticed a very faintly pink 'sludge' or a bloom once or twice in my water tank for the PAP machine, but that is only twice in eight years of use. It's only visible if you bother to peer at the inside of the tank AFTER you have discarded the dregs down a sink. It shows up better in good light, but with no water to speak of left in the tank. Even so, I refill my tank about every fourth day due to my particular settings. There's nothing wrong with emptying and refilling every night, even sterilizing the interior. Whatever floats yer boat. I don't, but when I do fill it every four days, I add some tap water, swirl, drain, and repeat. On the third time under the tap is when I fill it completely. This rinse disperses any concentration of minerals and insoluble organic materials that might eventually encourage that pink bloom.

For cleaning the tank every so often, you can use Scrub Free, or tile cleaner for bathrooms, toilet bowl cleaner, Javex, or any bleach, industrial strength vinegar (5% acetic acid, which some house owners use regularly for surface cleaning instead of bleach), or use the bisulphite. Just rinse well before restoring the properly filled reservoir to the PAP machine.

Yes, the metabisulphite is white and crystalline. Looks like table salt. One heaping tsp per 750 ml, or three cups of plain tap water. BTW, if you have decent tap water, and can drink it from the tap, that's all you need to use in your PAP machine. I have done so for eight years now. The secret, again, is to rinse really well before refilling the tank each time. You want to dilute the dissolved and non-soluble stuff in the dregs as much as possible to discourage that pink bloom.