A clear, calm rescue routine for a tub that has gone green. Most are recoverable in 24 hours without draining if you follow the steps in order.
A green hot tub is alarming but rarely fatal to your weekend. Most have lost their sanitiser temporarily, allowed algae or biofilm to take hold, and need a proper rescue rather than a refill. Here is the routine that works, in the right order.
Test pH first. The water must be at 7.2 to 7.6 before you shock. Add pH increaser or decreaser as needed and wait an hour with the jets running.
Test alkalinity. Anything below 80 ppm needs alkalinity increaser, again with an hour to settle.
Until pH and alkalinity are in band, the shock will not have its full effect. Skipping this step is the most common reason a green tub stays green.
Dose chlorine shock to 10 to 15 ppm. Pre-dissolve granular shock in a bucket. Pour in slowly with the jets running on full and the cover off.
Leave the cover off for one to two hours. The water will go cloudy white before it clears. That is the chlorine doing its work on the dead organisms.
Replace the filter at this point if it has not been changed in over a year. A failing filter cannot keep the dead matter out of circulation, and the tub will re-cloud overnight.
Run the jets continuously for at least 4 hours after the shock. Most modern tubs allow a continuous filter override mode for this.
After the water has cleared visibly (usually 4 to 8 hours), add a clarifier per the bottle. The clarifier clumps tiny suspended particles together so the filter catches them.
The next morning, rinse or replace the filter again, retest everything, and bring free chlorine into the standard 3 to 5 ppm bathing band before any use.
If TDS is above 2,000 ppm, if the tub has been green for more than a fortnight, or if there is a visible scum line that wipes off as a thick film. Otherwise the rescue routine is faster than a refill.
Yes. The cause is almost always insufficient sanitiser between sessions, often during a holiday. Get a friend or a service company to check the tub once a week if you are away for more than ten days.