There are several reasons that a spa foams. A common cause is the rapidly moving water combined with bathers bringing in soaps and detergents on their bodies and bathing suits. Showering before entering the spa helps prevent this problem. If excess detergents are the cause of the foam, using a defoamer such as a Rendezvous FOAM OUT, Leisure Time FOAM DOWN or Robarb SUDS-X will reduce the amount of foaming.
Another cause of foaming is soft water. This means that the calcium level in the water is low. This can be corrected by raising the calcium level to 150-400 ppm.
Organic waste build up can also cause foaming. By shocking the spa on a regular basis, the wastes are oxidized out of the water and foaming is reduced.