There are 2 types of polishing 1 uses a high flow rate to blast the tank walls. This requires that the outlet is moved around the tank to scrub the tank walls. (like hosing down a driveway). This type of polishing is done occasionally (every couple of years) and requires opening up the tank to work well. It is usually done by a commercial operator, but you can DIY.
The second type of polishing is done often (maybe everyday) is lower volume and works best when the tank is agitated by boat movement .
In my reading and experience both types of polishing systems can work well.
Commercial operators can only offer the first type of polishing and often imply this is the only option.