Aggressive Water
Aggressive (corrosive) water has an LSI below -0.
Definition
Aggressive (corrosive) water has an LSI below -0.3, meaning it is under-saturated with calcium carbonate and will dissolve minerals from pool surfaces and equipment.
Typical Values: LSI below -0.3 indicates aggressive water; below -0.5 is concerning; below -1.0 is a serious problem
In Plain Language
Aggressive water attacks calcium from plaster surfaces, etches concrete, corrodes metal equipment, and strips grout. The primary causes are low pH, low calcium hardness, and low total alkalinity. Fresh soft water (low mineral content) is inherently aggressive. The fix is raising pH, alkalinity, and/or calcium hardness until the LSI reaches the acceptable range of -0.3 to +0.3.
Why It Matters
Aggressive water causes permanent surface damage and equipment corrosion. Early correction prevents expensive repairs.
Typical Values
LSI below -0.3 indicates aggressive water; below -0.5 is concerning; below -1.0 is a serious problem
Last reviewed: 2026-06-01