Pool water turns brown due to high iron levels. Water from certain sources, such as wells, contains dissolved iron that oxidizes readily and turns brown when in contact with air. Acidic pool water, on the other hand, deteriorates metal equipment in the pool, introducing excess iron into the water.
Brown pool water is fixable through shocking, which is also known as super-chlorination. Chlorine oxidizes the iron and turns it into rust, causing it to settle out of the pool water. The oxidized metal is then able to be vacuumed out of the water. Another option is to add sequestering or chelating agents that are generally marketed as "metal out" products to the pool. These products combine with iron to form inactive complexes that cannot be oxidized by chlorine.