If the foam keeps coming back no matter what you add, the cause is in the water and the pipework. Here is how to break the cycle for good.
Short answer: stubborn foam is a build-up of soaps, lotions, deodorants and body oils dissolved in the water. Anti-foam knocks it down for a few hours, but if the residues stay in the water and the pipework, the foam simply comes back. The lasting fix is to remove the residues, not just the bubbles.
Every soak adds a little detergent to the water. Swimwear washed in normal detergent is the biggest single culprit, followed by moisturiser, fake tan and hair products. The jets whip these residues into foam, and because they are dissolved in the water, no amount of skimming removes them.
There is a second hiding place people miss: the pipework. A greasy film builds up inside the pipes over months, and every fresh fill picks some of it back up. That is why a tub can foam within days of a complete water change.
Stage one, today: a small dose of anti-foam knocks the surface foam down in minutes so the tub is usable. Treat this as a first-aid measure rather than the cure.
Stage two, this week: add an oxidising shock with the jets running to break down the dissolved residues, then a clarifier so the filter can strip out what remains. Rinse the filter after each of these, because it is doing the heavy lifting.
Stage three, next water change: run a plumbing flush through the pipework before you drain. This lifts the film inside the pipes so the fresh fill starts genuinely clean. If your foam always returns after a refill, this stage is the one you have been missing.
Rinse swimwear in plain water rather than washing it with detergent every time. Ask bathers to shower before a soak if they have applied lotion or fake tan that day. Keep your sanitiser in range, because a well-sanitised tub breaks down oils faster than a neglected one.
The foam itself is not usually harmful, but heavy foam means a heavy load of residues, and that load consumes your sanitiser quickly. Treat persistent foam as a sign the water needs attention.
You can, but you will use more and more of it as the residue load grows, and the water quality underneath keeps declining. Anti-foam plus a proper clean-up beats anti-foam alone every time.
Only if you flush the pipework first. Draining and refilling without a flush usually brings the foam back within a week or two, because the film inside the pipes reseeds the fresh water.