Calculator
Reef Water Change Calculator
"Do a water change" is the universal forum answer — but nobody tells you how much. A 20% change removes exactly 20% of your nitrate, no more. Enter where you are and where you want to be, and get the real plan: one big change vs. a series of smaller ones, with gallons and the level after each step.
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The math (shown, as always)
A water change of p% replaces p% of the water — and therefore removes exactly p% of anything dissolved in it. After one change your level is current × (1 − p); after n changes it's current × (1 − p)ⁿ. That's why repeated small changes have diminishing returns: the second 20% change removes 20% of what's left, not of the original.
One big change or several small ones?
Chemically they get you to the same place — the difference is stability. A very large change (50%+) can swing temperature, salinity, and alkalinity fast enough to stress coral, so match your new saltwater carefully or split the work across a few days. For most tanks, a series of 20–30% changes over a week or two is the calm way to knock a high reading down.
The assumption that matters
This is pure dilution math — it assumes your tank isn't producing more nitrate or phosphate between changes. Real tanks are. If your levels creep back up after every change, water changes are treating the symptom; the fix is export: better skimming (size yours here), or carbon dosing for nitrate and phosphate together.
What this costs in salt
Every gallon you change is a gallon of new saltwater. Once you have your plan, the salt cost calculator turns those gallons into a real dollar figure across 12 salts — the difference between brands is bigger than most people expect.
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