Bromine granules are the fast-dissolving form of bromine sanitiser, ideal for hot tubs. Learn how they differ from tablets, how to dose, and when to shock.
Bromine is a sanitiser that has been used for decades to control bacteria in spas, hot tubs and swimming pools. It comes in two main forms: slow-dissolving tablets for a floating dispenser, and fast-acting granules. Bromine granules are the loose, quick-dissolving version you add directly to the water, and they are a favourite for hot tubs where the water is warm and used often.
Tablets are designed to release slowly, holding a steady background level over days. Granules dissolve within minutes, so they are perfect for two jobs: giving the water an instant top-up before a soak, and re-activating spent bromine after you shock the tub. Many owners run tablets in a dispenser for the day to day level and keep granules on hand for a quick boost.
Bromine has three qualities that make it well suited to a British hot tub:
The trade-off is that bromine is not stabilised against sunlight, so it is less suited to open-air pools. For hot tubs, which usually sit under a cover, that is rarely an issue. For a full comparison, read chlorine versus bromine.
Aim to hold bromine between 3 and 5 ppm in a hot tub. To dose:
Always add granules to the water, never the other way round, and keep the lid closed firmly between uses. Our guide to testing hot tub chemicals walks through getting an accurate reading.
When bromine kills contaminants it becomes spent, forming what is called a bromide bank in the water. Shocking the tub with a non-chlorine shock reactivates that bank, turning spent bromide back into fresh, working bromine. This is why a regular shock keeps a bromine tub economical: you are recharging sanitiser you have already paid for rather than only adding more.
Bromine is a strong oxidiser, so treat it with respect. Keep granules in their original, tightly sealed container, in a cool, dry place away from direct sunlight and out of reach of children and pets. Never store them next to chlorine products or acids, and always use a clean, dry scoop, because even a trace of another chemical in a damp scoop can react. If granules ever clump or change colour, replace them rather than risk an uneven dose. Following the biocidal product guidance on the pack keeps both you and your tub safe.
Usage depends on how warm you run the tub and how often you bathe. As a rough guide, a well-covered four-person tub used a few times a week gets through a modest amount of granules each week once a tablet dispenser is carrying the baseline level. Warmer settings, more bathers and leaving the cover off all raise demand. Keeping the cover on between uses is the single biggest thing you can do to make a tub of granules last longer, because it stops sanitiser venting and slows evaporation.
Neither is better; they do different jobs. Tablets hold a steady level with little effort, while granules give an instant, controllable boost. Most owners use both together.
Yes, but the cleanest way is to drain, refresh and start the new sanitiser in fresh water. Mixing the two in the same tub is not dangerous in the water, but never mix the products in their containers or scoops.
Bromine is often the preferred choice for bathers who find chlorine drying or irritating, because it is milder and less pungent. As with any sanitiser, keep the level in the recommended band and shower after a soak.