Spas and hot tubs are like swimming pools in that they need chlorine to control the level of algae and bacteria in them. The chlorine releases some of the oxygen on the water molecules, which then kills these organisms to prevent them from growing in the water. It's important to maintain the correct amount of chlorine in your hot tub because if the level is too low, the spa won't be sanitary, and if it's too high, the excess chlorine will dry out your skin, which can be painful.
Test your spa water at least once a week using test strips. Your chlorine levels should be between 3 and 5 parts per million (ppm). To use the test strips correctly, follow the directions on their package. It will tell how long to dip the strip into the water and how to interpret the color.
Add spa chlorine when chlorine levels are too low (2 ppm or less), using the amount necessary to raise the concentration to at least 3 ppm. Add 1 oz. of granular chlorine per 40 gallons of water to raise the chlorine concentration by 1 ppm. Don't worry if you add a little too much because the concentration can be as much as 5 ppm, and overshooting the mark just means you won't have to add any more chlorine for a while.
Run the spa pump for about 10 minutes to mix the chlorine into the water. If you're not sure you've added enough chlorine, test the water again after the spa has finished circulating to be sure that you've brought the level to at least 3 ppm. If the new test shows that the spa is still low, add more chlorine as required. After chlorinating your spa a few times, you should get a good idea of how much chlorine to add each week, and you should get to the point where you'll get the chlorine to the right level on your first try.