How big should my weekly water change be?

10 % is the standard baseline. Reefs with heavy coral consumption sometimes go to 15–20 %. Bigger changes work but you're paying for diminishing returns — going from 10 % to 20 % cuts nitrate by an extra 10 %, not double.

Will a water change crash my cycle?

No. Bacteria live on every surface — rock, sand, sump, plumbing — not in the water column. You can change 100 % of the water without disrupting the biofilter. The shock to fish from the change is the real risk, not bacterial loss.

How do I lower nitrate fastest?

Multiple smaller changes over days, not one massive change. Three 20 % changes spaced 3 days apart drop NO₃ further than one 50 %, with less livestock stress. Or — fix the source: reduce feeding, increase skimmer, add carbon dosing.

Calculator

Water Change Calculator

Water changes are dilution. If your tank reads 15 ppm nitrate and your new salt mix is 0 ppm, a 20 % change leaves you at 12 ppm — not zero. This calculator runs the same math for alkalinity, calcium, magnesium, salinity, nitrate, and phosphate so you know what to expect before you dump the bucket.

Water change

Tank volume (US gal)

Change volume (US gal)

Tank values

Alk dKH

Ca ppm

Mg ppm

NO₃ ppm

PO₄ ppm

Replacement-water values

Alk dKH

Ca ppm

Mg ppm

NO₃ ppm

PO₄ ppm

Change %

20%

Predicted Alk

8.80dKH

Predicted Ca

428ppm

Predicted Mg

1304ppm

Predicted NO₃

12.0ppm

Predicted PO₄

0.160ppm

Pure dilution: new = old × (1 − x) + replacement × x. Assumes no biological draw-down during the change.

How this is calculated

x        = change_gal / tank_gal     // fraction replaced
predicted = current × (1 − x) + replacement × x

This assumes instant, complete mixing and no biological draw-down during the change. In practice your real result for nitrate and phosphate will be slightly worse than predicted (biology starts adding them back the moment the change is done) and slightly better for alk/Ca/Mg (these track new-water levels closely if you mix fresh).

Above 50 % single changes the dilution math is still correct, but acclimation shock starts to dominate. Coral don't appreciate a 50 % salinity / temperature / pH swing in 30 minutes — even if all three readings end up "fine".

FAQ

How big should my weekly water change be?: 10 % is the standard baseline. Reefs with heavy coral consumption sometimes go to 15–20 %. Bigger changes work but you're paying for diminishing returns — going from 10 % to 20 % cuts nitrate by an extra 10 %, not double.
Will a water change crash my cycle?: No. Bacteria live on every surface — rock, sand, sump, plumbing — not in the water column. You can change 100 % of the water without disrupting the biofilter. The shock to fish from the change is the real risk, not bacterial loss.
How do I lower nitrate fastest?: Multiple smaller changes over days, not one massive change. Three 20 % changes spaced 3 days apart drop NO₃ further than one 50 %, with less livestock stress. Or — fix the source: reduce feeding, increase skimmer, add carbon dosing.

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Sources & references

01
Randy Holmes-Farley — Optimal parameters (target salt-mix values)
https://www.reef2reef.com/threads/optimal-parameters-for-a-coral-reef-aquarium-by-randy-holmes-farley.173563/
02
Global Seafood Alliance — Typical chemical characteristics of seawater
https://www.globalseafood.org/advocate/typical-chemical-characteristics-of-full-strength-seawater/

Last reviewed 2026-05-25