Check the Conditioner/Stabilizer level in Pool
Please read AQUATIP 1 first. Before I had a salt system installed for my pool, I had no idea what conditioner was all about. I do now. Conditioner, aka Stabilizer or Cyanuric Acid to Pool Supply folks, is in the chlorine tabs and chlorine powder sold at pool supply stores. It is already in the store bought chlorine because pools need conditioner to prevent algae from growing in the hot summer months.

The conditioner acts like an umbrella protecting the chlorine from the effects of the sun. As I discovered a few summers ago, if the conditioner level in the pool gets too low, here comes algae, and quick. The recommended range in my salt system owners manuals is 60 - 80 ppm. You can check the conditioner level yourself as shown in the pictures to the left. It is very easy and an interesting test to perform.

However it probably is best to rely on a water sample analysis to be performed by a Professional Pool Supply Store.