I have known a person or two who needed to topoff their tank and didn't have any water in their storage containers, so they just ran a line directly from ro/di to the tank and turned it on, planning to turn it off later before they overflowed. I don't think either of theirs did, but I've overflowed too many buckets in the past to trust myself on that method. I've got a 20g AGA tank I use as my topoff reservoir with a Tunze osmolator in it. Awesome little topoff setup with laser siting, float switch backup safety sensor, and emergency shutoff of the unit if it runs too long so as not to overflow the tank.