Many times foam is caused by products being brought into the spa on our bodies. (i.e. Lotions, Make-Up, Hair Products, Perfumes, etc..) However there are two culprits that cause foam more than the rest: Unbalanced water and swim suits.
Unbalanced water can cause foaming, especially if your calcium hardness is too low. So staying on top of your water maintenance is very important in helping prevent this.
As for swim suits, it’s not necessairily the swim suit causing the foam, but the detergent being used to clean the swimsuit. Many times clothes put in the laundry do not get all the detergent out that is used when washed. So when the swimsuit is put back into the water with jets running this causes the leftover detergent to spread into the spa water causing foam to form on the surface. The best way to help this is to have a suit dedicated just for the hot tub. Only clean the suit when absolutely needed and try to use as little detergent as possible. Many clients will hang dry their suits after use and only wash in rare instances. And when they do wash, they will typically run an extra rinse cycle on the washer to rinse out any excess detergent that may be lingering in the suits.