Your sanitiser is the single most critical pool chemical, but pH is what makes it work. Here is how the two fit together, with UK dosing guidance.
Ask ten pool owners and you will get ten answers, but there is a clear winner: your sanitiser is the single most critical pool chemical. Without a stable residual of free chlorine or bromine in the water, nothing else you do matters, because the water is simply not safe to swim in. Everything else, the balancers, clarifiers and algaecides, exists to help the sanitiser do its job.
That said, the honest answer has a twist most guides miss. The most critical chemical is your sanitiser, but the thing that decides whether that sanitiser actually works is your pH. Get that relationship right and pool care becomes simple.
Chlorine and bromine both kill bacteria, viruses and algae by oxidising them. In a UK outdoor pool, free chlorine is the workhorse. We recommend holding free chlorine between 1 and 3 ppm for a domestic pool, tested at least twice a week and daily in warm spells.
Fast-dissolving chlorine granules are ideal for topping up the level, while stabilised tablets in a floating dispenser hold a steady residual between doses. If the level ever drops to zero, the pool is unprotected and can turn green within a day during a British summer.
Here is the part that catches people out. Chlorine at a pH of 8.0 is only about a quarter as effective as the same chlorine at a pH of 7.2. You can pour in sanitiser all day, but if the pH is high, most of it is wasted.
Keep pH between 7.2 and 7.6. Use pH minus (sodium bisulphate) to bring it down and pH plus (soda ash) to lift it. This is why we tell customers that a tub of pH minus is often the best value chemical they will buy: it makes the sanitiser they already own work far harder.
Two more chemicals protect that sanitiser and pH partnership:
Bromine is the more common choice for hot tubs than pools, because it copes better with warm water and is gentler on the skin. It is a perfectly valid sanitiser for a pool too, though it costs more and is not stabilised against sunlight, so it suits indoor pools best. For most UK outdoor pools, chlorine remains the practical pick.
Most of the sanitiser we see wasted in UK pools comes down to three habits. The first is chasing the chlorine level while ignoring pH, so every dose underperforms. The second is letting stabiliser drift too high, which locks up the chlorine you add. The third is dosing at midday, when strong sun burns off a good part of the chlorine before it can work. Dose in the evening with the pump running, keep pH in band, and test stabiliser once a month, and you will use noticeably less product across a season.
No. UV and ozone reduce the sanitiser demand, but they only treat water as it passes through the unit. You still need a residual of chlorine or bromine in the body of the pool to protect swimmers between passes.
Test sanitiser and pH at least twice a week, and every day in hot weather or after heavy use. A simple test strip is fine for routine checks; a liquid test kit is more accurate when you are chasing a problem.
Test pH first, then sanitiser. Cloudy water is very often high pH letting the chlorine underperform. Bring pH back to 7.2 to 7.6, top up free chlorine, and the water usually clears within a day.