Fiberglass Pool, CSI drop, how important


Gold Supporter
May 19, 2021
Charlotte, NC
Pool Size
Salt Water Generator
SWG Type
Pentair Intellichlor IC-20
According to my research, and manufacturer warranty information, etc. I know that even though I have a FG pool I still need to manage CSI but with a little less criticality. I am not closing, but the water is too cold to swim.

Does negative CSI impact any equipment or the gelcoat in a negative way? I have been trying to be in the -.3 to -.4 range to reduce SWG scale, and it seems to have worked, but now I am not using SWG for the winter.

That said, my numbers have been pretty stable, but my pH crept to about 8, and I targeted 7.5 with an MA dose. But then my reasonably negative CSI dropped a lot. I haven't done much to bring TA up, but if I plug in a higher TA it doesn't show much impact to CSI either. I could add some CH, but before doing that, I thought I would ask the pro's here. I hadn't checked CH in about a month, and it also has dropped by 25 from 275 to 250. Playing with Pool Math, even bringing CH up to 350 makes little impact to the CSI number.

Before adding the MA one week ago my numbers were:
FC: 8
CC: .5
pH: 8
Temp: 58F
CSI: -.43

Current chemistry:
low csi.jpg

Texas Splash

Mod Squad
TFP Expert
LifeTime Supporter
Jun 22, 2014
Texas, San Antonio/Marion, South-Central Area
Pool Size
Salt Water Generator
SWG Type
CircuPool RJ-45 Plus
CSI is not applicable to your equipment and should have no bearing on your gelcoat unless you have a waterline tile. It's primarily a tool for plaster management. That said, it is common for the pH to rise in cold water and it helps to counter-balance the cold water's effect on CSI. So ever though CSI doesn't apply to you, I would allow the pH to rise to the 7.8 - 8.0 range. No need to fight the natural effects of cold water, and no need to add calcium or adjust the TA at this point.
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