First off, welcome to R2R.
#WelcometoR2R
As stated, there are no dumb question here. We've all been in your shoes not knowing what's up. So ask away, and if you wish to attract a special group of folks that can help, simply include
#reefsquad , and they will be notified of that post.
To answer your question, yes, storing water is just fine . Many of use use Brute (Rubbermaid) trash cans to store and make saltwater, but many other types of containers are possible. I know many small tank people who use 5 gallon buckets. Pretty much anything made with polyethylene (HDPE or LDPE, Recylce 2 or 4, respectively) would be reef safe.
Other plastics are reef safe as well. I make my water in five gallon drinking jugs and then use them to dump into a 32 gallon Brute to make saltwater, and also feed my ATO (Automatic Top Off) reservoir, which looks similar to a water cooler dispenser.
One more thing with your RO/DI unit....you mentioned moving it to the garage.....you didn't mention where you lived, but if it's in an area that has freezing temperatures, don't make water when it's freezing or leave it in the garage when not using it. If it freezes, those canisters pop off from the expanding ice easily.
So ask away with any other questions you might have, and again, WELCOME!
