A good water change is more than pulling the plug: the film inside the pipework goes with the old water only if you flush it first. Run Hot Tub Flush before you drain to purge the pipework, deep-clean the filter while the shell is empty
Purges the biofilm from the pipework before you drain
Deep-cleans the filter cartridge while the tub is empty
Lifts the waterline tidemark and cleans the empty shell