Inflatable spas have smaller volumes, weaker heaters, and PVC liners. The chemistry rules adapt accordingly. Here is what to do differently.
Inflatable hot tubs (Inflatable Spa, Cleverspa, MSpa, and similar) have become hugely popular in the UK garden. The water chemistry is broadly the same as for a hard-shell tub but with three differences: smaller volumes (typically 600 to 900 litres), PVC liners that do not tolerate harsh chemistry, and weaker heaters that take much longer to recover from a temperature drop.
Inflatable tubs are typically 600 to 900 litres. That is half the volume of a typical hard-shell. Almost every chemical dose is half what you would use for a regular tub.
Misreading the bottle as if it were a 1,500-litre tub is the single most common cause of overdosed inflatable spas.
Always pre-dissolve granular products in a bucket and add slowly. The smaller volume means localised concentration is much higher than in a bigger tub.
PVC tolerates lower pH and lower chlorine peaks less well than acrylic. Keep pH between 7.2 and 7.6 (a tighter band than the 7.2 to 7.8 used for acrylic).
Never shock above 8 ppm. The PVC starts to bleach at sustained 10 ppm.
Use bromine if you can. The peaks are gentler and the PVC stays in better condition for longer.
Avoid copper-based algaecides, which can stain the liner permanently.
The 1.5kW or 2kW heater on a typical inflatable can take an hour to recover 1C of temperature loss. That changes how you handle the tub.
Keep the cover on whenever you are not bathing, and only run the bubbler when you are in the tub (the bubbler is a heat-loss machine).
Do not bathe more than two people at a time in a typical four-person inflatable. The volume is too small to absorb the load without the chemistry crashing for the next session.
Same chemistry, smaller doses. Buy hot tub chemicals rather than pool ones. Brand-specific Inflatable Spa or Cleverspa branded products are usually the same chemistry at a higher per-kilo price.
Every 4 to 6 weeks for moderate use, much shorter than a hard-shell. The smaller volume means TDS climbs faster.