My dealer recommends Spa Sentry when you fill, but I found out that it makes a huge mess if you put it in first thing. I don't know what is in that stuff, but in addition to locking in the PH, it takes all kinds of cr@p out of your water. It must have clarifier, softener, metal out, and who knows what else. I was wiping the sides of my tub for 30 minutes after puting it in, and it took me forever to get my filters clean afterward. The scum line was gross, but I'm glad I got all that stuff out of my water.
What I do now is wait a day or two after changing the water so I can balance the PH and TA and filter everything filterable out of the water. Then I add the Spa Sentry, and it doesn't make as big a mess. It really locks in the PH though, don't put it in unless you have it right in the range.