Have you heard of Cyanuric Acid?
Hypochlorous Acid (the form Chlorine takes in pool water) breaks down in the presence of sunlight, then the Chlorine gases off in to the atmosphere resulting in low Chlorine levels in your swimming pool.
Cyanuric Acid is an incredible invention that bonds with Chlorine and helps retain it in the pool water. It works by it’s incredibly effective ability to absorb UV rays (sunlight) and so the Chlorine does not break down. Conversely, the presence of Cyanuric Acid lessens the effectiveness of Chlorine, and too much of it can cause the Chlorine to become ineffective and require the pool to be drained. So don’t over do it. Get a professional.
Although Cyanuric Acid is an Acid, it has a relatively small effect on the pools ph, unlike Muriatic Acid or Hydrochloric Acid which have a much larger effect depending on the alkalinity.