Spa scale can be caused by a few different things: hard water (high Calcium), water that is highly alkaline, water with high iron, and body oils, lotions or other contaminants introduced into the water by bathers. it can also be a sign that you are using too much lithium sanatiser.
First you will want to check your water balance with a 4 in 1 test strip. Ok range for pH is between: 7.2 - 7.8 Ok range for Total Alkalinity is between: 80 – 160 Hardness between 100- 250 PPM's If you have hard water, deposits of calcium in the form of a light film or an actual hard scale called lime scale can develop on your spa shell.
You may also want to use a spa water softener. Acid cleaners can remove film and scales; one of the least expensive and natural products you can use to remove calcium deposits is white vinegar, a mild acid. You can use this with a non-scratch pad. If your spa water is balanced yet you are still getting a scale build-up you may need to increase your circulation time so that your filters have time to do more work. It is also recommended that you clean your filters more often. Once per week is our recommendation.
Of course your filters need to be in good order. If they are older than twelve months it is time to change them. Additionally to help reduce scale and maintenance you can use BioGuard Aquality which controls pH, Total Alkalinity, softens spa water and prevents metal staining.