Yes, with the ACE system you don't want to have more calcium in the water. To deal with foaming, it would be better to use de-foaming products assuming they do not contain calcium. I suppose this is one downside to the ACE system -- that one has to keep the CH low to prevent scaling in their salt cell, but to do so raises the likelihood of foaming. Making sure you thoroughly rinse your swimsuit to remove any soap will help as will making sure you are fairly clean before getting into the spa (i.e. it's not a bath).
MPS doesn't play any role at all because you need not use any of it. You can use chlorine alone to maintain the spa. The
ACE manual only talks about MPS in the context of being supplemental oxidation, but they refer to it as with "Dichlor or MPS". It is true that their "Hot Spring Freshwater Spa Care Kit" includes MPS and you can certainly use it until it's gone, but there's no need to buy any more after that since it's far more expensive than bleach (and even more expensive than Dichlor). As for dosing, if the ACE system were not handling any of the bather-load, you could figure that every person-hour of soaking would need 3-1/2 teaspoons of Dichlor or 5 fluid ounces of 6% bleach (3-1/2 fluid ounces of 8.25% bleach) or 7 teaspoons of non-chlorine shock (43% MPS). Of course, the ACE system will handle part of the bather load so you'll need to just test and adjust to figure out what you need to do.
Note that MPS will increase the salt level as well. Remember that I said that for every 10 ppm FC added by bleach that it increases salt (sodium chloride) by 17 ppm. Well, it takes 7.26 fluid ounces of 6% bleach to get to 10 ppm FC in a 350 gallon spa and it would take 2.33 ounces weight of non-chlorine shock (43% MPS) to be equivalent in oxidizing capability for bather waste. The MPS also results in salt though it results in potassium sulfate plus you need to add a base to raise the pH or baking soda to raise the TA since MPS is net acidic. So in effect you end up with the equivalent of 3KNaSO
4 + K
2SO
4 which in this example is 2.58 ounces weight of salt or 55 mg/L. In other words, every 10 ppm FC equivalent with bleach gives you 17 ppm salt as sodium chloride whereas when using MPS you get 55 ppm salt as potassium sodium sulfate. So the buildup of salt is much faster when using MPS, though the salt does not build up chloride and builds up sulfate instead. This means that the increased salt from bleach may result in more production of chlorine from the ACE system whereas the buildup of salt from MPS may result in more oxygen species. This shouldn't matter very much.
As far as corrosion risk, the faster salt buildup from MPS would be potentially more damaging and having more sulfates in the water is not good for stainless steel when there is already a lot of chloride in the water. So, bottom line, I would stick to bleach and don't use the MPS beyond what they gave you in the startup kit.
