CSI Assistance


New member
May 3, 2017
The Woodlands, TX
Hello TFPF,
Question RE CSI. I'm using the Pool Math app and currently have a CSI of -0.57. The app gives a warning that anything below -0.3 could be corrosive to tile. I have a chlorine pool and my current numbers are as follows:
FC - 3.5
pH - 7.5
TA - 70
CH - 275
CYA - 50
BOR - 30
CSI - -0.57
Water Temp: 55 (F)

I've adjusted various numbers around trying to determine how best to get CSI up but I'm not having any luck getting CSI up to -0.3. Any thoughts on what I'm missing?


Last edited:


Mod Squad
TFP Expert
LifeTime Supporter
Jul 7, 2014
Bedford, TX

Welcome to TFP... a great place to find the answers to all of your pool maintenance questions... :shark:

Simply let your pH sit at 7.8, gets you less than -.3... Bring your CH up to 350 also helps...

Most pools like to have a pH at the high end.. A pH of 8.0 or less is just fine.


Jim R.
Likes: gibby


Mod Squad
May 3, 2014
Laughlin, NV
I suggest you not over-react. The CSI is low mainly due to low temperature. I would suspect in a week or two your pH will rise some and that will also push your CSI up.

The process of plaster degradation is not instantaneous. If you kept your CSI that low for many months or years you would have a problem. But for the short period your water is cold, it is not a 'panic' issue. Test your pH next week. It should rise a bit, especially if you run your spillover. Do not drop your pH below 7.8 until you need to due to rising TA or higher water temperatures.
Likes: gibby


New member
May 3, 2017
The Woodlands, TX
Thanks mknauss. Not necessarily panicking, but was curious as to what I should do. We had actually just lowered pH from 7.8 using acid recently. I did not realize water temp played that significant of a role in that calculation. Our water temp in the summer is usually around 85-89 and that temp moves CSI up to -0.2 with all other values left alone. So what I'm hearing is allow higher pH values in the winter when water temps are cooler?
Apr 15, 2018
Daphne Alabama
I had similar problems the other week, and I threw some calcium chloride in it to raise CH, CH was at 350 today, pool temp 54f and PH 7.9

CSI = - 0.05

That calcium harness plays a huge role in the csi, in pool maths you can play with figures to see how to balance to get the right csi