CSI only applies to plaster pools. Hot tubs/spas are generally fiberglass. Also, there is the assumption that spa/hot tub water gets replaced every few months. CSI is not a factor for stand alone spas/hot tubs.
I was thinking if you happened to get a positive CSI you'd start scaling, but after playing with poolmath I realized you'd likely need a fill CH of like 500+ for this to really start being a concern, since your TA will almost certainly have to always be right around 50.
The assumption is spas/hot tubs are covered and you need some way to burn out the natural CCs that accumulate from its usage. MPS is a way to do that.
I suppose this depends on the usage. I haven't seen an issue with CC buildup with our inflatable spa, though we generally aren't using it every single night, and don't keep it very hot. I have seen CC start to build up on the occasions we use it every night, and so I can see how with heavy usage (daily long, hot soaks) this could be a factor. Alternatives also include opening an outdoor spa to the sun or adding a UV system.
It wasn't really the mention of MPS that bugged me, it was that it said to add MPS every week, rather than adding if you had high CCs. Adding stuff based on testing, not blindly, is what TFP is all about, right?
Spas/hot tubs get a lot of use for the water volume and this ensures the water stays sanitary.
But if you're regularly testing FC and CC, you shouldn't need to "shock" on a regular basis. Again a generic calendar based series of chemical additions as opposed to testing based additions is what bugs me on that sticky.
Obviously you generally need to add chlorine or another oxidizer (such as MPS) after each use, to oxidize waste created during the soak, which isn't something you do in a pool, but this was mentioned clearly in the sticky and isn't the same as a calender based "shock". Adding after each soak is what will ensure it stays sanitary, not a once a week addition.
Dry acid is not recommended for pools due to the buildup in sulfates. Sulfates can damage SWG's, plaster and concrete. You don't have these issues in a fiberglass standalone spa/hot tub that replaces water every few months. So dry acid can be used in hot tub/spas and some folks find it easier handling and safely storing dry acid.
I see. I was under the impression that sulfates were bad for metals, and so could damage heaters since they have a metal core,
such as in this post. In fact, you yourself said they can increase likelihood of corrosion in metals
in this post. Would that not include the metals in heaters found in spas? It appears it's more for SWCG plate corrosion, but many people will add a SWCG to their standalone spa (I plan to), and so shouldn't use MPS.
It is mentioned in the wiki -
www.troublefreepool.com
I always forget about those articles. Seems that that could use some additional information too, especially as it's located in "further reading", which to me means "more advanced", but that might just be me.
Write up your techniques and let's see how it works. There are many ways to keep a spa/hot tub sanitary and safe.
I will certainly do this once I get at least one water cycle (2-3 months) in our first "real" spa at our upcoming new house, which will likely have far heavier usage than our current inflatable spa has ever had.