White scale on SWG plates is a indication of calcium scale. What causes it is pool water that is not correctly balanced. More importantly, you need to understand that the conditions inside the SWG cell are not at all comparable to the bulk pool water. The pH inside the cell is typically much higher than the pH of the bulk pool water because of hydrogen gas generation. So, even if your pool water numbers look balanced or about right based on pool store recommended ranges (which, by themselves, are not typically correct), they can be way off with respect to the SWG cell. Calcium scale forms when the pH, TA and CH are too high. High temperatures (like those found in a spa) exacerbate the scale problem.
The bigger problem is that it is hard for us to help you because no one on TFP believes pool stores numbers are accurate. They can sometimes get pH and chlorine correctly measured but they are often dead wrong when it comes to CH, TA and CYA. So while the pool stores tell you that your CH is only 250ppm, chances are they are way off.
The trick for reducing scale inside a cell is to ensure that you calcite saturation index (CSI) is slightly negative. But, you can't calculate the CSI without good test values. And those good test values only come when you measure your own pool water. You can plug the numbers you have in PoolMath (you need to know your pool volume) and see where your CSI is at. Then you can report back and let us know if the number is positive or negative.
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