Acrylic Spa?
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I would say it depends on how hard the water is to start out with. Why don't you measure the Calcium Hardness (CH) and Total Alkalinity (TA) of the water and post those numbers. The only downside to the hardness would be if it is so hard that you would end up precipitating scale (calcium carbonate) by the time you adjusted your pH (and possibly TA). So unless your CH is sky high to begin with, you will probably be OK, but post your numbers and I can let you know.
If the CH is too high, then yes, you can use your water softener to substitute sodium (or potassium) for the calcium which will reduce the CH. If the TA is too high, then can just aerate the water (i.e. run the spa jets) for a while and acid to keep the pH low (around 7.2 should be fine) and the TA will drop (i.e. you would follow Ben's Lowering Your Alkalinity procedure, but wouldn't go quite as low in pH in order to avoid any damage to your acrylic and also because you shouldn't need to since you've got an outstanding source of aeration with your spa jets).
As for the rest of the water chemistry, you pretty much run it similar to what you would do in your pool with a few exceptions. First, your quantities will be measured in teaspoons and tablespoons. Second, you probably won't need to add any extra Calcium, but could keep the water roughly balanced by just using a mix of fill water with softened water though that is not necessary (you don't have gunite/plaster/grout exposed to water). Third, the aeration will cause the pH to rise significantly so you will want to keep your TA low -- down around 70 or 80 ppm. Fourth, if you use chlorine as your sanitizer, then you'll want to use Dichlor a few times, but only enough to add at least 10 ppm but not more than 20 ppm of CYA. The amount of Dichlor that gives you 1.0 ppm FC will also give you 0.9 ppm CYA and since the CYA test doesn't measure low levels well, you'll just have to keep track of how much Dichlor you add. Typically, if you add enough to raise the FC by 4 ppm, then you would just add it about 3-4 times. After that, you would add unscented bleach (until you empty and refill your spa after a few months and start over with some Dichlor again). Keep your FC at 4 ppm minimum, so you'll need to start out higher (perhaps 8 ppm) each time you add chlorine. The combination of 20 ppm CYA and 4 ppm FC should be sufficient to kill the not-so-easy-to-kill bacteria Pseudomonas aeruginosa which causes "hot tub itch". You can't go without using any CYA or else your chlorine will outgas much too quickly and will also be too harsh on your swimsuits. One tip: open up the spa for 5 minutes before using it -- that will dissipate any accumulation of clean chlorine smell.
As for shocking, that's something that I'm not sure about in a spa and you could really help us out with a little experiment. I'm assuming you already have an accurate drop-based FAS-DPD chlorine test, so please keep track of your Combined Chlorine (CC) level (BEFORE each time you get into the spa) and see if just maintaining the Free Chlorine (FC) levels stated above keeps the CC in check (i.e. below 0.5 ppm). If it doesn't, then you will need to do regular (possibly weekly) shocking. You can first try shocking just with bleach raising the FC to 10 times the CC level and keeping the spa cover off for at least an hour. If this doesn't seem to work, then you might need to use a weekly maintenance dose of non-chlorine shock (potassium monopersulfate, KMPS), but will then not be able to check for CC after that since the KMPS gets reported as CC unless you get the expensive reagent (K-1520 $19) from Taylor that removes such interference.
Richard
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