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Total Dissolved Solids (TDS) in a spa or hot tub should not exceed 2500 ppm, or 1500 ppm over the level naturally occurring in the fill water. If it is higher than this, draining and refilling with water of significantly lower TDS would be appropriate. The following formula is a good rule of thumb for estimating when to drain the spa:

Spa volume in litres/(10 x average daily bather load) = number of days between drains