Your chlorine level dropping that quickly means that the chlorine is working on something in the water, getting used up. Increase you dose of chlorine untill you oxidize the contaniments and start maintaing a chlronie reading. Metals in the water will chew up chlorine as well as risdual soaps from swimsuits ect. Have you tried a clarifier and then cleaning your filters super well to remove any contaniments/waste from the tub? I see in other posts you have problems with smells ect. Also a sign of contaniments. Strong chlorine smell means not enough chlorine (usually) as strange as it sounds. It is working to hard. Mildew smells and body oder smells mean not enough chlorine, stuff is growing. Even if you are clean when using the tub, each person sweats about a pint per spa use...this will cause BO smell if there is not enough sanitizer to kill the microorganisms. As stated in another post, PH is important for keeping the chlorine working right as are the CYA levels.