Short answer: Wellis water care products keep hot tub and swim spa water balanced between water changes: the Wellis Crystal system, pH and alkalinity products, cleaners, chlorine tablets, bromine salt and test strips. Sauna Dekor hands them over with the tub and shows the routine Wellis publishes; the products are Wellis's.
Hot tub water is warm and shared, so it is tested and corrected on a schedule. Wellis publishes the dose and the interval on each product page; the products here are listed with those figures where Wellis gives them.
For the tubs themselves see the Wellis overview; filters, covers and steps are under Wellis accessories.
Wellis SwimSpa chemical set
Wellis Chemical Set in.Clear
Wellis Spa Cleaner
Wellis Filter Cleaner
Wellis pH down
Wellis Alkalinity balancer
Wellis Metal clear
Wellis Foam down
Wellis Bright and clear
Wellis Chlorine tablets
Wellis Bromine salt
Wellis Pool check 5 in 1