The salt could rust out a metal container and leech toxic stuff in the water. Go to home depot or lowes or a local hardware store and get a 5 gallon paint bucket. Without the paint
And the size container you need depends on how much salt water you are mixing up. How big is your tank? If you are needing ALOT, you prolly need a 30 gal Rubbermaid trashcan, with a powerhead for stirring and a heater. If you are only mixing 1 gal at the time, just use an empty milk jug (cleaned out very well).