Continuous build up of calcium carbonate is usually caused by either high levels of calcium or alkalinity in the water column or potentially low magnesium.

Ideally the calcium and alkalinity levels will need to be between 380-440 ppm  and 8-10  dkh respectively. Calcium or alkalinity levels that are out of balance or higher than this can encourage precipitation of calcium carbonate. Equally magnesium levels that are low will allow unwanted precipitation, correct magnesium of around 1350ppm - 1500ppm will help prevent calcium carbonate seed crystals from forming.

If you are dosing either calcium or alkalinity components also be careful not to drip solutions near the intake of the skimmer. This will cause blockages quickly between the air water interface of the venturi and will precipitate on warm pump components such as the impeller bearings.