On the advice of others (and some reading) I purchased 2 grey brute trash containers. One is 32 gallons and the other is 44 gallons. I use the 44 gallon container to mix saltwater, and the 32 is used to make a large batch of kalkwasser that is dosed upstairs to the reef over the course of a month with a litermeter III dosing pump.
No problems with this setup. I used cheaper containers before, and there was often a film on the surface, no matter how much I cleaned the containers. The brutes are foodsafe (FDA approved and stamped on the bottom... certain colors though), and rugged. I fill the 44gallon one about 3/4s (or more) and it doesn't seem to deform.
I found that simplifying my waterchange and topoff routine has helped my tank more than any other change over the years. I now do a weekly waterchange with ease, and because it's simple, It actually gets done.
I also use a sterilite container plumbed with a bottom bulkhead that sits on a shelf in the fishroom for "miscellaneous" RO/DI... ie: a bit for turbo calcium when needed, a bit for culting rotifers and phyto... It's very handy.
Just my input. It's worth repeating: streamling your setup will make you life easier, and will allow you to carry out routine maintenance with ease.
Just my input.
R.