We recommend a hot tub drain/refill at least twice per year, once every six months. Heavy hot tub usage will call for more drain/refills. Believe it or not, your hot tub water will become “tired”. Tired hot tub water means that the total dissolved solids (TDS) has reached its max and in the industry known as “high TDS”.
I like to use the chocolate milk example. When you make a glass of chocolate milk, you can add and add cocoa powder. So much that the powder no longer dissolves and it drops to the bottom of the glass. The milk has reached its TDS max and will not allow anymore powder to dissolve.
The same thing happens with hot tub water. After you add inorganic materials such as sanitizers, pH/Alk decreaser/increaser, foam down, metal gone, non-chlorine shock, deodorant, lotions, sunscreen, bug repellent and organic materials such as sweat, urine, etc. the water will become tired and have high TDS. You may also notice a fizz pop effect on the surface of the water. It can be an unpleasant coughing experience.
But I don’t use my hot tub a lot, do I still need to do a drain/refill every 6 months? The answer is yes. As long as you are doing proper hot tub maintenance and adding chemicals to balance the water, it’s adding to the TDS.