I have an in-ground plaster pool, app 25,000 gal. Using a Taylor K-2006 test kit (as well as another less expensive kit, just for comparison) I've watched my Total Alkalinity drop over the last several weeks. It used to be a little over 100 -- perhaps even as high as 120 -- but now it's down around 70 to 80. I assume this is probably because I have to add muriatic acid once a week or so in order to keep the pH down. I'm not sure why the pH creeps up, but I'm told that it could be due to the "relatively" new plaster (about a year or so old), as well as the fact that my water supply has a higher pH (at the top of the color test scale on a Taylor K-2006.) The consistent addition of the acid, however, would seem to explain the lowering TA.
What I'm not clear about is whether or not action needs to be taken. I've read over this forum, along with various other pool info web sites, and I see totally contradictory advice on this. Some people say that it's crucial to raise the TA to 100 using baking soda, and that not doing this could cause damage to the pool surfaces and equipment. Others, however, say that keeping TA in this lower range is actually advised in order to combat the rising pH condition. I have noticed that as the TA has dropped, the rising pH has slowed, which would seem to concur with the later response. If this is detrimental to equipment, however, I would obviously want to raise the TA. I also read that higher CYA levels can inflate the TA test, and given that my CYA is 70 to 90 or so, this would imply that my TA is even lower. Others say that CYA in the low 100's is just find, and does not need to be factored in to the TA results.
The net result is that I'm totally unsure what to believe, so I'm looking for any feedback I can get on this.
My general levels are as follows:
FC 3 (using Trichlor tablets and liquid chlorine to raise/shock as needed.)
pH 7.5 (rises to 7.8, then I lower it to 7.4 using liquid muriatic acid)
CH 375 (my water source is fairly hard)
TA 70 to 80
CYA 70 to 90
Thanks for any feedback on this,