Covered by not air tight is best. You want to keep dirt and dust out and you don't want an excess of carbon dioxide mixing in with the water. The carbon dioxide will alter the chemistry of the water. That is why using aeration is a big "NO!" for use as a 'mixer.'
Something like this works great for mixing salt...Without the blue frame stand, mine was only about $50. Mine has a loose fitting flat lid that just rests on top (not making a seal) and allows for powerhead cords or tubing. I think they make a version with a cone shaped bottom so you can draw off the sediments before you add the water to the tank.
I have two of them: one for freshwater make up (RO water) and one for mixing salt. The gallon marks on the side make it nice when adding dry salt to the water. The next step for me is semi -automating the process so I can draw 5 gallons of water out of my sump and then add 5 gallons from my mixing tank back to the sump all with switches.
My philosophy is make it easy to use so you are more likely to do regular water changes
Becareful when you are using those trash can, some of them are not made with food-graded plastics and might leak chemicals back out, especially if you feed RODI water directly into them.
I've been looking for a while now and I can't seem to find the calculator for temperatures affect on Specific Gravity. I'm mixing new water and my heater I use for mixing water broke so I want to have it as close to my desired salinity as possible before putting a heater in it in the morning and then making any minor adjustments necessary.
I have a Brute gray plastic trash can. Is this suitable for mixing water? The Brute brand is mentioned above but I just wanted to be sure that the gray colored ones are OK to use.