SEALED container?

[Aelvion]

then mixing process to properly exchange gases???

[lakshwadeep]

It's okay, but you may want to aerate it before adding it to the tank. Note that covering while mixing can result in your water overheating because of the powerhead's heat.

[farkwar]

Well, I use a 5 gallon jug that I shake to mix mine.

 

Note that doing so without a lid will get water all over the carpet.

[gregzbobo]

If your powerhead has provisions for it, connect some airline to it that has the other end outside the container, that will aerate the heck out of it.