o-guy,
You are correct when you say unbuffered MPS will bring pH down.....it's pH is between 3-4 so it will do a pretty good job of dropping pH if that's what your water balance needs. I just changed my water on T-giving Day. Out of the tap, it's right at 7.3-7.5 but after a day or two of introducing air into the water, the pH will rise a bit--usually up to about 8.0 or so. Since I've got such great water out of the tap, I've only introduced dichlor so far. At the end of this week, I'll shock it with straight MPS which will cause the pH to settle in nicely around 7.5 or so. Down the road, I'll buffer the MPS with soda ash depending on current pH when I shock. Since dichlor is about 7.0, it too will cause pH to drop, albeit only slightly.
I've purchased mine from both Doc's online store and from a local pool dealer. Brand shouldn't make a difference--just look for 100% MPS (Potassium Monopersulfate) on the label.
The only drawback would be TDS buildup that is quicker than using dichlor to shock but I have found this to be no problem at all with 3-5 month water changes.
The advantage to MPS is being able to use the tub within about 30 minutes after shocking. As a side note, I also like the aroma of the water after an MPS shock.....it reminds me of freshly starched and ironed shirts......a very old memory since I haven't seen or smelled a freshly starched shirt in years thanks to permanent press and laziness.
