I use two different size holding tanks. One is 65 gal. the other is 35 (maybe 40 if you fill it all the way to the rim).
So obviously, I use the larger one to store RODI water, from there, I transfer it over to the smaller container where I mix up approx. 30 gal of saltwater at a time. 30 gallons of saltwater is good for 2 water changes.
I'm so glad I finally got around to finishing this project. Life & kids take up so much of my time. I probably bought those containers over a year ago.