By The Zagosa Team. Published .
Reviewed or updated .
Foam appears when surfactants (body oils, sun cream, shampoo and detergent left on costumes) meet the aerated water your jets throw up. It is a nuisance rather than a hazard, but it is also the visible warning of an organic load that is grinding your sanitiser down.
What is really behind it
In UK tubs, foam usually traces back to one of four things. Detergent left on swimwear that went through the wash with ordinary powder. Body lotions and hair products rinsing off in the warm water. Low calcium hardness, which lets the surface tension fall too far. Or a tub simply overdue a refill, where Total Dissolved Solids have crept past the point the water can carry the load.
Clearing it inside a day
Resist the urge to empty a bottle of defoamer straight in. Defoamer knocks the bubbles down but leaves the cause floating and clogs the filter. Keep it for emergencies, like guests turning up within the hour.
For a proper fix within 24 hours: shock the water with a non-chlorine oxidiser, run the jets for 15 minutes, then pull the filter and rinse it under a hose. Brush the waterline while you are there. Together that clears most foam by the next morning.
Keeping it from returning
Rinse swimwear in a basin of hot water before every soak, no detergent. That one habit does more to prevent foam than anything you can buy.
Hold calcium hardness in the 100 to 250 ppm band. Low calcium is a quiet driver of stubborn foam.
Deep-clean the filter every fortnight in a proper filter soak, not just a hose rinse. Oils work their way into the pleats and leak back into the water on every cycle.
Common questions
Is it unsafe to bathe in foamy water?
Not directly, but it is telling you the sanitiser is working harder than it should. Test the free chlorine or bromine before you get in, and if it is below the recommended band, top up first.
Will a drain and refill sort the foam?
Usually, particularly if the tub has passed the 12-week mark. Pair the refill with a pipe purge so you do not carry the same biofilm straight back into clean water.