My go-to is using black Plasti Dip spray. I clean the side of the tank very well with alcohol to get rid of any oils from fingers, manufacturing, etc. I then spray about 3-5 coats of the Plasti Dip. This makes a light rubber coating that can be removed like taking a rubber glove off if you were to change the location of the tank or sell it in the future. The more coats, the easier it is to peel off.