If you have a pH of 7.8, a TA of 50 ppm, a borates of 50 ppm and a CYA of 40ppm(?), I'm assuming you'll have a very low CA? And thus, you'll have little pH drift from CO2 off-gassing during periods of aeration?

Are you / they adding dichlor (CYA) to spas because they are uncovered outdoor spas, or to reduce DPB formation? I get the "don't let CYA build up" thing.

So, I can see strategy of high pH + low CA + elevated non-CA TA + CYA control as a reasonable effort.

Can those results be improved by using that strategy, except switching to a very high CYA plus very high Cl residual? Something like 200ppm CYA plus 25 ppm Cl as a normal level? That might overcome the problem of insufficient total chlorine residual (in 300 gal water) to react with the bather goop of a couple of spa users.

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I've also realized I don't have any idea of what the 'normal' spa configuration is. In my area, even today, most spas are detached. But, I gather that attached spas may be more the norm in California and Florida.

- Are most of these in a shared equipment / unshared water configuration?
- Spill-over spas have to be in at least partially shared water configuration. Is it typically 100% shared, or partially shared?

I'm going to need to see if I can find some piping and control layout diagrams for these spas, to see have they've been design and should operate.