The 12-week rule, where it comes from, and the honest reasons your tub may need a refresh sooner if you live in a hard-water UK area.
The standard guidance is every 12 weeks for an average household, with the maths coming from a simple ratio: roughly 1 in every 3 litres of your tub gets bather-loaded each quarter. If you use the tub more than three times a week or run a busy household, drop that to every 8 weeks.
The PWTAG calculation is litres divided by ten, divided by daily bathers. So a 1,200-litre tub used by two people daily has a refill window of about 60 days. Most UK households are nearer 90 days because the tub gets two or three uses a week, not seven.
The rule keeps Total Dissolved Solids (TDS) below the level where water starts to feel slick, refuses to balance, and chews through sanitiser even when the readings look right.
The water smells off within a day of shocking. That is usually TDS or organic load past the point where chemistry can recover.
The pH refuses to settle. You add the correct dose, retest the next morning, and it has drifted again. Fresh water is the cure.
Foam appears the moment the jets run. A defoamer hides it for a session, but the underlying problem is buildup of body oils and product residues.
Run a pipe-cleaner overnight before you drain. The biofilm in the lines is what reseeds bacteria into the new water within days, and a pipe purge product knocks it loose.
Drain to a soft soakaway or a foul drain, never to a watercourse. The neighbour's lawn is not a soakaway.
Refill through a hose-end pre-filter if you live in a hard-water postcode. It removes the metals that cause green tinging and saves you from a calcium battle from day one.
Yes, but every six months rather than every twelve weeks is fine. Stagnant water still develops biofilm even if no one is using it.
After. A pre-holiday refill is wasted water if the tub then sits unused for two weeks. Refresh on your return so the water is at its best when you actually use it.