Phosphates are a common concern for pool owners, especially during Houston’s hot summer months. They are nutrients that algae can use to grow, and they may enter a swimming pool through rainwater, lawn fertilizer, leaves, soil, swimmers, fill water, and certain pool chemicals. A phosphate reading does not automatically mean that your pool has a serious problem. However, when phosphate levels become elevated, they can make algae more difficult to control and increase the amount of chlorine needed to keep the water clean. What Are Phosphates? Phosphates are compounds containing phosphorus. They are usually invisible in pool water and do not directly make the water unsafe. The concern is that they can serve as a food source for algae. Chlorine is still the pool’s primary defense against algae. A properly sanitized pool may remain clear even when phosphates are present. However, high phosphate levels can reduce the margin for error.