To help with your water changes, buy a Brute food grade trash can and lid. Then add a power head, heater, and thermometer. It's best to premix your saltwater a day or two in advance of a partial water change. This way the water is the same temp and salinity as your display tank.
When it's time for a PWC
, I simply add 10-15 gallons of RO
water, turn on the power head, then add my salt. The power head will essentially mix your water for you. Then let it sit overnight with the heater and power head on.
Here's a pic if my homemade setup.