Pool tablets and hot tub tablets are not the same chemistry. Here is what each contains, and why mixing them up causes a slow disaster.
Most large swimming pool chlorine tablets are stabilised trichlor (trichloroisocyanuric acid). Most hot tub tablets are unstabilised dichlor or, more commonly in the UK, bromine. Putting trichlor pool tablets into a hot tub is one of the most common avoidable mistakes we see, and it is hard to undo.
Trichlor adds cyanuric acid (stabiliser) every time it dissolves. In a 30,000-litre pool the build-up is gradual. In a 1,200-litre hot tub the same tablet adds the same CYA, and the level climbs to 50, 80, 100 ppm within weeks.
Above 50 ppm CYA in a hot tub, free chlorine becomes increasingly sluggish. You can have 5 ppm reading on the test strip and still get bacterial issues, because most of that chlorine is bound to CYA and not available.
Either small (20g) bromine tablets in a floating feeder, which are the UK norm, or fast-dissolve dichlor granules added by hand. Both are designed for the small volumes and high temperatures of a hot tub.
Slow-dissolve hot tub chlorine tablets do exist but are less common in the UK. If you choose them, get the ones marked specifically for hot tubs and check the CYA monthly.
Test cyanuric acid. If it is above 50 ppm, dilute by partial drain and refill (replace one third of the water).
Do not switch to a non-stabilised chlorine the same week, because the existing CYA will continue to bind your chlorine until it is diluted out.
Next refill, switch to bromine or to dichlor, depending on your preference. Keep the pool tablets for the pool, where they belong.
Most pH adjusters, alkalinity increasers, and non-chlorine shocks cross over fine. Chlorinated and stabilised tablets do not. When in doubt, check the active ingredient on the bottle.
Same chemistry, fine to use. Hot tub-branded versions are often the same product in a smaller bag at a higher per-kilo price.