What you are saying is, the pool water is intermittently shared with the spa, not continuously. That doesn't change anything. I've been doing this ( PoolForum, etc) for over 20 years. I'm well aware that this is NOT information either spa owners OR spa sellers want to hear. But the fact that the facts are unwanted doesn't make them 'not the facts'.

You can go lots of other places, and get lots of answers telling you how wonderful spas are, and how well they can be sanitized. And, you can believe what you like. What you won't be able to do is find a pool service person who will get into your spa! A spa sales person probably would -- in order to make the sale -- and then go take a shower as soon as you sign on the line.

Meanwhile, my advice remains . . .

1. Nuke it.
2. Flush it.
3. Start over.
4. Use it ONLY with trusted family members!

Why is it foamy, THIS time, when it hasn't been foamy BEFORE? I don't know, and honestly, I don't care.

I'm not going to spend time trying to 'fix' what's intrinsically broken: spas are unsanitary unless drained REGULARLY. If they are not CONTINUOUSLY part of the pool, the 'spa = toilet bowl' rules still apply. Trying to INTERMITTENTLY mix spa and pool water is a poor idea, foisted on consumers by builders who don't understand pool and spa chemistry . . . or who don't care. There are a number of chemicals used in pools -- stabilized chlorine, stabilizer, flocs, most algaecides, etc -- that should NOT be in spas, and which WILL cause problems in spas. In addition, it is NOT possible to 'balance' water (pH, alkalinity, calcium) for BOTH a pool and a spa: different values are required, for a number of reasons. (In fact, if you are putting algaecide into your pool . . . and pool water into your spa . . . that would definitely result in foaming!)

But you want to smoke? Your choice.
You want to be fat (like me, unfortunately)? Your choice.
You want to use a typically managed spa? ALSO, your choice.

But all those choices have consequences.

You want me to tell you, it's all OK? That's my choice, and I won't do it. Sorry.