saltwater mixing/storage

[5ft24]

I am assuming freshly mixed saltwater needs to breathe...
Mixed up 50 gallons (used RO/DI water). added about 4 teaspoons of Seachem reef buffer to get the PH up to 8.3 to match my tank (did this with the last batch as well with no issues...)
this time, because of the dust and some bugs that dropped in last time It sat in the open barrel, I took a plastic trash bag and put over the top and weighted it down to keep it from blowing off when the garage door was opened/closed. left the heater and pump in the tank keeping it circulating...
Pulled the plastic off this morning, and everything in the barrel had a slimy white film on it. Tested the water, and the PH was 7.0! calcium tested out at 400 and salinity at 1.024...
It was 8.3 after the buffer, calcium was 450, and salinity was 1.0255 right after the buffer dissolved.
the plastic had the barrel sealed... no fresh air in or old air out.
I put 2 airstones in the tank, one pump in the bottom moving the water, and another pump on the bottom with the outlet about a foot above the water splashing back down...
PH is coming back up... it's about 8.1 now, but Calcium and salinity are the same...
I don't have a magnesium test, and with the way things changed and whatever precipitated out, I don't know if I want to use this batch even if PH comes up to 8.3...
Anyone else seen this happen/Advice??
Thanks

[HighlandReefer]

Provided your salt mix has the proper levels of alk, calcium and magnesium you don't need to adjust its pH. The pH of fresh salt mix is dependent on the amount of CO2 dissolved in the water and will equalize quickly once the CO2 reaches equilibrium in your tank.

Regarding the white film on the surface, it could be bacterial or it may be a calcium carbonate film. Either way you could skim it off and use the water IMHO. You don't want your salt mix contaminated with bugs and other things from the surrounds. If you use a salt mix with organics in it like RC, the organics can feed bacterial growth. ;)

[bertoni]

I suspect the pH dropped because bacteria were consuming the nutrients and oxygen from the water, and producing carbon dioxide. I'd probably pitch the water at this point, but it might be okay.

[5ft24]

Now that it has been aerated all day, the white stuff doesn't feel slimy, just chalky... still not completely comfortable and would rather spend a few bucks more on RO/DI water to mix a new batch than take a chance...