Foam is almost always organic build-up, not faulty chemicals. Here is what causes it, what fixes it in 24 hours, and what permanently keeps it away.
Foam is what you get when surfactants (body oils, sun cream, shampoo, detergent residues from costumes) meet aerated water from the jets. It is annoying rather than dangerous, but it is also the visible tip of an organic load that is wearing your sanitiser down.
Most foam in UK hot tubs is one of four things. Detergent on swimwear that was washed with normal washing powder. Body lotions and hair products that wash off in the warm water. Low calcium hardness, which lets the surface tension drop too far. Or simply a tub that is overdue a refill, where Total Dissolved Solids have crept past the point where the water can carry the load.
First, do not just pour in a bottle of defoamer. Defoamer collapses the bubbles but leaves the cause in the water and gums up the filter. Use it as a temporary measure only when guests are arriving in the next hour.
For a real fix in a day: shock the water with a non-chlorine oxidiser, run the jets for 15 minutes, then take the filter out and rinse it under a hose. Brush the waterline. The combination clears most foam by the next morning.
Rinse swimwear in a basin of hot water before each use, no detergent. That single habit cuts foam more than any product on the shelf.
Keep calcium hardness in the 100 to 250 ppm band. Low calcium is a quiet cause of persistent foam.
Deep-clean the filter every fortnight in a proper filter soak, not just a hose rinse. Oils embed in the pleats and re-enter the water on every cycle.
Not in itself, but it tells you the sanitiser is working harder than it should. Test the free chlorine or bromine before you get in. If it is below the recommended band, top up before bathing.
Usually yes, especially if the tub has gone past the 12-week mark. Pair the refill with a pipe purge so you do not reintroduce the same biofilm into clean water.