By The Zagosa Team. Published .
Reviewed or updated .
Opening the cover to green water is a nasty surprise, but it is rarely the disaster it looks like. In almost every case the sanitiser has simply run flat, algae or biofilm has moved in, and the tub needs a proper rescue rather than a full refill. Work through the following in order and you will usually have clear water again within 24 hours.
Get the chemistry ready first
Start with pH. It needs to sit at 7.2 to 7.6 before any shock goes in. Add pH increaser or decreaser as required, then run the jets for an hour to let it settle.
Now check alkalinity. If it reads under 80 ppm, top up with alkalinity increaser and give it another hour to stabilise.
Shocking before pH and alkalinity are in range is the single biggest reason a green tub stays stubbornly green. Do not skip this part.
Hit it hard with chlorine
Bring chlorine shock up to 10 to 15 ppm. Dissolve granular shock in a bucket first, then pour it in slowly with the jets on full and the cover off.
Keep the cover off for one to two hours. Expect the water to turn a cloudy white before it clears, which is the chlorine breaking down the dead organisms.
If the filter has not been swapped in the last year, change it now. A worn filter cannot hold back the dead matter, and the water will simply cloud over again by morning.
Circulate, then clarify
Run the jets non-stop for at least four hours after shocking. Most modern tubs have a continuous filtration override that is ideal for this.
Once the water looks visibly clearer, usually after four to eight hours, add a clarifier at the dose on the bottle. It gathers the tiny suspended particles into larger clumps so the filter can trap them.
The following morning, rinse or replace the filter again, retest every reading, and bring free chlorine down into the normal 3 to 5 ppm bathing band before anyone gets in.
FAQ
When do I just have to drain it?
Reach for the drain hose if TDS is over 2,000 ppm, if the tub has sat green for more than a couple of weeks, or if there is a thick scum line that wipes off as a heavy film. Short of that, the rescue routine beats a refill for speed.
How do I stop it happening again?
Almost always the trigger is the sanitiser running low between sessions, typically while you were away. If you are off for more than ten days, ask a friend or a service company to check the water once a week.