Algae blooms however, can raise the pH of your pool water. The process of photosynthesis can produce enough oxygen in the water to raise your pool pH levels, particularly if tested near the end of a warm, sunny day. High pool alkalinity levels will make it hard to adjust your pH level. To lower your total alkalinity, use pH decreaser. In most cases, additional treatments will be necessary if you need to lower alkalinity more than 30 or 40 ppm.