I like the idea of marbles mentioned in the earliest posts of this thread. But when I buy industrial sized stuff which I have to do, they used to come in collapsable bladders. So when they went to rigid plastic last year (they being Agfa) for there 5 gallon stuff, I went to a camping store and bought collapsable water bladders, they work great at keeping the air out of both stock and tank solutions when not in use. In a pinch once when I only made up a very small portion of chemistry for some testing, I used vinyl gloves filled with the about 500ml of solutions and rubber banded the wrists. This worked for over night but not for a long haul. Hi, btw, I'm new my name is Jill. Should have put that as my ID but I wasn't thinking I guess. Not too internet savvy, sorry.