my dealer has told me 3 reasons for foam:
1. soap, shampoo, lotion, etc.- use a foam down
2. ph is off balance - adjust ph into range
3. too much organic matter - use scum gon (an live enzyme that will eat that stuff out of the spa and get rid of the foam) they said that scum gon will extend the life of the spa water as well, i use it around the 2 month period and when needed