ph and alkalinity issue

[rick]

I've been getting rashy of late in my tub.   I use only dichlor to maintain and to shock, and my test kit monitors ph and chlorine.   Over time my tub's ph will go down and I am constantly using ph+ to get it back to 7.5

So I took a sample to my dealer the other day and my alkalinity was off the charts!   I think it was at 300!

So the fix was to slowly introduce ph- to my spa over a 2 week period to get it back down.   But I told him that my ph levels will take a serious hit doing this.   In the end I decided to drain and refill.

So now I've purchased those all in one test strips so that I can monitor alkalinity as well.   

So you see my issue,   if I'm always adding ph+ to get my ph level right,  this is in turn also raising my alkalinity to extreme levels over time.   What is the proper balancing act to keep ph and alkalinity good?

PS: I'm wondering if maybe it is because I never do 100% refills.  The last few months,  when I need fresh water,  I've been opening up the drain while at the same time refilling with the hose and I'll run this for about an hour.   Obviously, I'm not getting a 100% change over in my water but it just seemed easier and cheaper on the electric bill to do it this way.   I wonder if this method is causing my TDS to go high.

[chem geek]

You probably aren't aerating the tub enough to have the high TA raise the pH for you automatically via carbon dioxide outgassing.  If you run the jets, then you can have the pH go up with no change in TA.  You are right that the pH Up will increase the TA if you don't have enough aeration.  By aerating more, you should be able to get to a point where the pH is more stable, though the TA might be around 150 ppm or so.

If you can't aerate, then another alternative is to use a pure base (i.e. lye) to raise the pH, but unfortunately there aren't any spa products I know of that use that.  The next best thing would be to use 20 Mule Team Borax since that will raise the pH without increasing carbonates so raises the TA by half as much as pH Up.  However, if you use that a lot, then you may add too many borates -- up to 50 ppm cumulative is OK.