A hot tub's water should have a pH level between 7.2 and 7.8, with an ideal range between 7.4 and 7.6. Water with a low pH level corrodes the hot tub's metal parts, heater and the seals. Low pH also causes skin irritation. Hot tub water with low pH has a high acidic level. Before raising the pH level, test and adjust the water's total alkalinity -- total alkalinity controls and holds the water's pH level.
Run the hot tub's pump motor for 10 minutes, then turn it off.
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Test the hot tub's pH level with a water test kit 12 inches below the water's surface. Liquid test tests use droplets that change the color of the water sample. Test strips automatically change color when wet. Compare the color of the water sample or the test strip to the test kit's pH chart and find the water's pH level.
Look on the hot tub's identification tag, usually located near the pump motor, to find the amount of water the hot tub holds.
Determine the amount of soda ash (sodium carbonate) needed to raise the pH level. A chart on the soda ash's container has a column that lists pH levels and a row that lists hot tub size in gallons. The number where the hot tub size row and the tested pH level column meet represents the amount of soda ash needed.
Turn on the hot tub pump motor.
Broadcast the required amount of soda ash over the deep end of the hot tub. If the hot tub does not state the amount of water it holds, broadcast 1/2 cup of soda ash over the deep end.
Run the pump motor for one hour, then test the water's pH level with the test kit. Add more soda ash as needed.