What are Phosphates? Why are they in my Pool?
Phosphates or phosphorus in your pool without treatment will support the growth of Algae. Algae feeds off of phosphates and can have your pool become a not so pretty or healthy place to swim. See the informational video found here.
The main reason to drain a pool is usually high levels of Cyanuric Acid which is the substance that protects the chlorine from ultraviolet rays burning the chemicals off. We usually set the level at 50ppm chlorine pool/100ppm salt pool.
The problem of this level rising is caused by the need to prolong levels of sanitizer/chlorine in the water for a weekly maintenance adjustment of chemistry. This is usually done with tabs that are 50% cya adding 2 tabs/lb raises 50ppm usually in a pool season.
Poor water chemistry due to use of chems is needed to sanitize the pool and tap water which contribute to Total Dissolved Solids, which is the total measured water content of organic and inorganic substances. The other main reason is having high levels of Cyanuric Acid, which is one of the most common reasons for needing to drain a pool.