There are plenty of commercial cleaners on the market, but they can be expensive so here’s a cheaper way.
Fill the bath with hot water, add about 2 tablespoons dishwasher (not washing) powder and about half a cup of bleach and run the water jets for 15 minutes. T
hen drain the water and refill with cold water only. Run the whirlpool for another 10 minutes, then drain again.
You should do this about once a month to keep your pipes clear.