Before I used some chemical that was recommended by the bubble bath manufacturer. Now I use vinegar instead and it seems to work well. This is how I do:
Fill the bath tub with cold water until it covers all the jet stream holes.
Pour 5 deciliter vinegar into the water.
Run the jet streams for about five minutes.
Leave the bath for 20 minutes before you empty it.
Fill the tub with cold water again.
Let the jet streams run for five minutes.
Empty the bath and shower the inside to remove any residue.