By The Zagosa Team. Published .
Reviewed or updated .
Those big swimming pool chlorine tablets are almost always stabilised trichlor (trichloroisocyanuric acid). Hot tub tablets, by contrast, are usually unstabilised dichlor or, more often in the UK, bromine. Dropping trichlor pool tablets into a tub is one of the most frequent avoidable mistakes we come across, and it is a pain to reverse.
The trouble with trichlor in a tub
Every time trichlor dissolves it adds cyanuric acid (stabiliser). Spread across 30,000 litres of pool the build-up is slow. Drop the same tablet into a 1,200-litre hot tub and it delivers the same CYA into a fraction of the water, so the level climbs to 50, 80, even 100 ppm within weeks.
Once CYA passes 50 ppm in a hot tub, free chlorine turns increasingly sluggish. The strip can read 5 ppm and you can still hit bacterial trouble, because most of that chlorine is locked to CYA and not free to act.
The tablets a hot tub actually wants
Either small (20g) bromine tablets in a floating feeder, which is the UK standard, or fast-dissolve dichlor granules added by hand. Both are made for the small volumes and high temperatures a hot tub runs at.
Slow-dissolve hot tub chlorine tablets exist but are less common here. If you go that way, choose ones marked specifically for hot tubs and check CYA every month.
If you have already used pool tablets
Test cyanuric acid. Above 50 ppm, dilute it out with a partial drain and refill (swap around a third of the water).
Do not switch to a non-stabilised chlorine in the same week, because the CYA still in the water will keep binding your chlorine until it is diluted away.
At the next refill, move to bromine or dichlor to suit your preference. Keep the pool tablets for the pool, where they belong.
Common questions
Is any pool chemical fine in a hot tub?
Most pH adjusters, alkalinity increasers, and non-chlorine shocks cross over without trouble. Chlorinated and stabilised tablets do not. When unsure, read the active ingredient on the bottle.
What about pool calcium increaser?
Same chemistry, perfectly fine. Hot tub-branded versions are often the identical product in a smaller bag at a higher price per kilo.