Hot tub water foam is caused when the Total Dissolved Solids (TDS) level in the water build up and are brought to the water surface by either the use of the air blower or the air being injected through your jets. The dissolved solids in the water sort of "stick" to the air bubbles coming up through the water and are brought to the surface. You see this as "foam". The foam is actually the dissolved solids clinging to the air bubbles as they escape upwards into the air. The foam is the "stuff" that was previously dissolved in the water.
TDS levels can build up in the water from many things. When you get in a spa or hot tub, a lot of unpleasant stuff comes off of you. Perspiration, dead skin cells, underarm deodorant, soap residue and dirt from bathing suits, skin lotions and creams, hair spray, etc. Some of this microscopic stuff is caught by your filter, but a lot of it simply dissolves in the water. Also, every time you add any type of chemical to the water, you are introducing additional solids into the same body of water.
Depending on how much of these items are put into the water, and how fast (days, weeks, or months), this will determine the point at which your water becomes saturated. Once that starts to happen, the first thing you will see is the foam. Soon thereafter, the water will permanently cloud – as it reaches its true saturation point of TDS’s.
Once that happens, no amount of filtering or additional chemicals will clear the water and you will have to change it. A Water Defoamer will help combat foam for a while, but your best bet is to be as clean as possible before going into the tub. That way you should be able to get about 8 weeks between water changes.