What Water Balance Means
Water naturally wants to reach a chemical equilibrium with its surroundings. If your water is undersaturated with calcium, it will try to dissolve calcium from whatever it touches, like plaster walls or grout. If it’s oversaturated, it will deposit calcium as scale on surfaces, in pipes, and on equipment.
The Calcite Saturation Index (CSI) puts a number on this tendency. Zero means the water is neutral. Negative means it leans toward dissolving (aggressive). Positive means it leans toward depositing (scaling).
The Acceptable Range
| CSI Value |
What It Means |
| Below -0.7 |
Aggressively corrosive. Action needed. |
| -0.7 to -0.5 |
Slightly aggressive. Worth monitoring. |
| -0.5 to +0.5 |
Balanced. This is the goal. |
| +0.5 to +0.7 |
Slightly scaling. Worth monitoring. |
| Above +0.7 |
Actively depositing scale. Action needed. |
PoolFu treats the entire -0.5 to +0.5 range as equally acceptable. A CSI of -0.49 is not meaningfully different from 0.0 in practice. There’s no bonus for hitting exactly zero.
What Goes Into CSI
Five factors determine your CSI: pH, total alkalinity, calcium hardness, water temperature, and total dissolved solids (TDS). Change any one of them and the balance shifts.
PoolFu also applies a correction for cyanuric acid. CYA contributes to your measured alkalinity reading, but it doesn’t actually participate in the carbonate chemistry that CSI measures. Without correcting for this, pools with high CYA would appear more balanced than they really are. PoolFu uses the Wojtowicz method for this correction, which accounts for both pH and temperature. It’s more accurate than the common shortcut of “subtract a third of your CYA.”
Your Surface Matters
CSI means different things depending on what your pool is made of.
Plaster and pebble surfaces are calcium-based. Aggressive water (negative CSI) will gradually etch and dissolve the surface over years. For these pools, keeping CSI near zero or slightly positive is worth the effort.
Vinyl and fiberglass surfaces aren’t affected by aggressive water. A negative CSI won’t harm your liner. Your equipment still appreciates balanced water, but there’s no surface damage risk. PoolFu adjusts its messaging and urgency accordingly.
Spas have a unique challenge. See the temperature section below.
Temperature Changes the Target
This is where most pool apps get it wrong. The standard CSI target of 0.0 assumes moderate pool temperatures. Hot water precipitates calcium much faster than cool water. A spa at 104°F with a CSI of +0.1 will scale up noticeably, even though +0.1 looks fine on paper.
PoolFu adjusts CSI targets based on your water temperature:
| Water Body |
Temperature |
Target CSI |
Acceptable High |
| Pool |
Below 60°F |
-0.1 |
+0.3 |
| Pool |
60–85°F |
0.0 |
+0.5 |
| Pool |
85–95°F |
-0.1 |
+0.3 |
| Spa |
95–100°F |
-0.2 |
+0.1 |
| Spa |
100–102°F |
-0.25 |
0.0 |
| Spa |
102–104°F |
-0.3 |
0.0 |
| Spa |
Above 104°F |
-0.35 to -0.4 |
-0.1 |
For spas, the target moves progressively negative as temperature rises. Running a spa at CSI 0.0 and 104°F will coat your jets and heater with calcium scale. PoolFu targets slightly negative CSI in hot water to prevent this.
Expected Chemistry for SWG Pools
If you have a salt water generator with CYA in the 60–90 ppm range, your CSI will naturally run negative. This is expected.
The reason is mathematical. CYA at those levels reduces the effective carbonate alkalinity by 20–30 ppm when the Wojtowicz correction is applied. That shifts the CSI calculation downward. The water isn’t actually aggressive in any meaningful way. It’s behaving exactly as the chemistry predicts for your equipment setup.
Most apps would flag this as a corrosion warning and tell you to raise calcium or pH. Following that advice would push other parameters out of range while fixing a problem that doesn’t exist.
PoolFu recognizes this pattern. When your SWG pool shows negative CSI with CYA in the expected range, the app explains that this is normal for your setup instead of generating unnecessary recommendations.
How PoolFu Fixes CSI When It Needs Fixing
When CSI is genuinely outside the acceptable range, PoolFu looks for the most practical adjustment. The decision follows a clear order.
pH first. It’s cheap, fast, and reversible. If your CSI is negative and your pH is at 7.4, there’s room to raise it toward 7.6, which improves CSI without going outside the ideal pH range. PoolFu checks whether this would cause a problem for your chlorine effectiveness before suggesting it.
Calcium hardness second, but only if it’s already outside the ideal range for your surface and moving it would also help CSI. PoolFu won’t suggest raising calcium in a vinyl pool just because CSI is slightly negative. That would solve a problem your liner doesn’t have.
Accept or dilute when no good lever exists. If your pH is already at 7.8 and your calcium is appropriate for your surface, there may not be a practical way to fix CSI without making something else worse. PoolFu tells you this honestly rather than suggesting changes that would create new problems.
One important exception: when dissolved iron or copper is present in your water, PoolFu will not suggest raising pH to improve CSI. Raising pH with metals present precipitates them into permanent surface stains. Metal treatment comes first. Once metals are handled, normal CSI optimization can resume. See Testing for Metals for the full picture.
The Bottom Line
Water balance is real, but it’s not the most important thing about your pool. Sanitizer and pH come first because they directly affect safety. CSI is a long-term surface protection metric that matters over months and years, not hours.
PoolFu weighs CSI at 10% of your Health Score. It evaluates it in context with your surface type, temperature, and equipment. And it never suggests a CSI fix that would create a parameter problem. Your water is one system. PoolFu treats it that way.
