TDS- Total dissolved solids are solids that can't be filtered. These solids are things such as suntan lotion, chlorine, dirt, pollen body waste, stabilizer and algaecides. A high TDS reading will likey mean that the pool remains cloudy no matter how much shock or clarifier is added to the swimming pool. Most swimming pools should be drained and refilled every 3 to 5 years. A normal TDS reading should be 450 ppm or lower. To simplify TDS, imagine an 8 oz. glass of water. If you add a tea spoon of baking soda and stir it, the baking soda will dissolve. If you keep adding baking soda to the same 8 oz. of water, eventually it will not dissolve and the water will appear cloudy. The cloud in the water is a measure of the total dissolved solids. To lower the TDS, the pool will have to be partially drained and refill until the TDS reading is 450 ppm or lower (the lower the better).