Q: Should I use 25% or 50% NaOH?

50% is cheaper to ship per pound but freezes below 54 °F. 25% stays liquid down to −13 °F and is the standard for outdoor day tanks.

Q: Can I use soda ash instead?

Yes — Na₂CO₃ adds alkalinity without raising pH as aggressively. Cost is higher per pound of alkalinity, but it's safer to handle than 50% caustic.

Q: Why not use lime?

Lime (Ca(OH)₂) works but adds calcium hardness. Soft-water systems generally prefer NaOH or soda ash, which don't add hardness.

Question 1

How much does 1 mg/L NaOH raise pH?

Accepted Answer

It depends on starting alkalinity. In low-alkalinity water (<30 mg/L) it can raise pH 0.3–0.5 unit per mg/L; in high-alkalinity water (>200 mg/L) the change is <0.1 unit.

Question 2

Should I use 25% or 50% NaOH?

Accepted Answer

50% is cheaper to ship per pound but freezes below 54 °F. 25% stays liquid down to −13 °F and is the standard for outdoor day tanks.

Question 3

Can I use soda ash instead?

Accepted Answer

Yes — Na₂CO₃ adds alkalinity without raising pH as aggressively. Cost is higher per pound of alkalinity, but it's safer to handle than 50% caustic.

Question 4

Why not use lime?

Accepted Answer

Lime (Ca(OH)₂) works but adds calcium hardness. Soft-water systems generally prefer NaOH or soda ash, which don't add hardness.

Caustic Soda Dose Calculator for pH & Alkalinity Adjustment