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To lower both total alkalinity and pH, you will need to add an acid. Liquid muriatic acid (hydrochloric acid diluted to 20 percent) and dry sodium bisulfate are among the most popular choices.  The acid combines with the water, thereby increasing the concentration of hydrogen ions and lowering the pH. Similarly, the acid reacts with bicarbonates in the water and reduces the water's total alkalinity in the process. You could also look for a generic “pH decreaser,” “alkalinity decreaser,” or “combination decreaser” chemical at a pool supply store. The pH will drop faster than the alkalinity, so you need to fix the alkalinity first. Once you balance the alkalinity, the pH will gradually adjust to it.  Always follow the instructions on your pH/alkalinity chemical when preparing the correct amount. As a general rule, you will need 1.6 lbs (725.75 g) sodium bisulfate or 1.3 qt (1.23 L) muriatic acid for every 10,000 gallons (37.85 kL) of water to drop the total alkalinity by 10 ppm. Scoop water out of your hot tub into an 8 gal (30.28 L) plastic bucket until that container is three-quarters full. Pour the full amount of pH decreaser into the water in the bucket and let it dissolve. You must add the acid to the water. Pouring the acid into the bucket first and adding the water can cause damage to the bucket and may result in ineffective mixing. Make sure that the pump and filters are running. The hot tub should be set at its usual temperature and speed before you continue. It is crucial, however, that no one is in the hot tub while you are balancing the water. Slowly pour the diluted decreaser into the center of the hot tub. Pour the acid in gradually instead of dumping it in all at once. Adding the acid too quickly can cause damage to the sides, bottom, and equipment in the hot tub. Allow the pump to circulate the water for three to six hours after you add the decreaser. During this time, the pumps should circulate the water and acid together more thoroughly. Only after the two have been well combined will the pH and alkalinity be consistent throughout the hot tub, and you need to wait until these measurements are consistent before moving on any further. Test the alkalinity first, then the pH.  If done correctly, the alkalinity should already be balanced. The pH may not be balanced yet, however. If either the alkalinity or pH is still high, repeat the process. Continue as needed until the water is well balanced. You should completely drain the water in your hot tub at least once every four to six months. Afterward, fill the hot tub back up, balance the pH and alkalinity as needed, and continue to monitor the conditions as usual.  You will need to balance the pH and alkalinity of your water nearly every week if you use the hot tub regularly. Adding chemicals to the water so often can cause excess build-up, and you will likely notice that it is becoming more difficult to balance your water conditions. Once you notice this difficulty, it is time for you to change the old water out for fresh water.
Choose the right chemical. Determine your initial measurements based on total alkalinity. Mix the chemical together with a small amount of water. Turn the hot tub on. Add the diluted chemical to the hot tub. Give the water a chance to balance. Test the alkalinity and pH again. Periodically drain the water.