This happens especially when chemicals intended for swimming pools are used for spa baths. But it's easy to avoid an overdose of chemicals (applies to chemicals in general, also chlorine-free) if you follow our guidelines!
Pool chlorine (in powder form or as tablets) are not suitable for hot tubs and the reasons for this is a different chemical composition which are not designed for use in a hot tub. The water in hot tubs is much warmer and hot tubs contain significantly less water. Using a floating dispenser is not a reliable solution as chlorine seeps out even if the dispenser is completely closed.
This often causes overdosage problems which can can damage the spa and equipment. Without a dispenser the tablet dissolves even faster in warm water than in cold pool water.
It's different in a pool because it has a significantly larger volume of colder water. This means a pool dispenser works as intended because the tablet dissolves more slowly over a larger volume of water, resulting in a significantly lower chlorine concentration. It seems obvious and easy to understand when explained, but most people do not consider it unprompted, which is why we want to bring it to your attention.
The left photo shows surface damage where a larger chlorine tablet (20 grams) has lain on the surface of the plastic material. The second picture shows bubbles developing in the plastic material after a long time overdose of chlorine.
The underside of covers can alse corrode as shown in the picture below. To the right is a picture of the "Chlorine Week Tab" which is for pools only!
We do not recommend chlorine levels above 1.5 ppm and that is approximately what is achieved with a 5 gram tablet.
We have conducted our own testing over a long time period. We have come to the conclusion that a good and safe way to keep a bathing water clean and fresh is to use CombiTabs 2-3/tablets per week (1 tablet every two to three days depending on how often the bath is used. If seldomly used, one tablet per week can be enough.) and Spa Tab 5 one tablet before and another after bathing. CombiTabs are used as the primary cleaning and Spa Tab 5 for "long-term effect".
Note that during bathing, chemicals dissolve faster and "evaporate" from the bath. With the lid on, higher levels are quickly achieved and under the lid vapors are formed which increases the concentration even more. This is also part of the reason why it is easy to overdose on chemicals, particularly when unreliable dispensers are used.