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.

Aquarium Water Change Calculator — Parameter Dilution Math

JavaScript is disabled in your browser, so the interactive calculator below won't run. The math, formula, and worked example are all written out further down this page under “How this is calculated.” Enable JS (or open this page in another browser) to use the live calculator.

Whether you compute by hand or in the calculator, the same caveat applies: verify against your test kit and the linked primary source before dosing. reefcalcs is reference information, not veterinary or engineering advice.

Water change

Tank volume (US gal)

Change volume (US gal)

Tank values

Alk dKHCa ppmMg ppmNO₃ ppmPO₄ ppm

Replacement-water values

Alk dKHCa ppmMg ppmNO₃ ppmPO₄ 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.

Gear for thisAffiliate links

Red Sea Coral Pro Salt — 22 kg bucket (175 gal mix)

~$90 · View on Amazon →

Rubbermaid Commercial Brute 14 gal container

View on Amazon →

Affiliate links. As an Amazon Associate, reefcalcs earns from qualifying purchases.

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.

Where to buy

Affiliate links

Salt mix in bulk for routine water changes. Mix in a 14-gal Brute can the night before, heater + powerhead overnight, refractometer-confirm to 35 ppt before dosing.

Red Sea Coral Pro Salt — 22 kg bucket (175 gal mix)Best per-gallon value for SPS tanks.View on Amazon →Tropic Marin Pro Reef Salt — 25 kg (200 gal)7–8 dKH spec. The go-to for calcium-reactor and Triton tanks.View on Amazon →Instant Ocean Reef Crystals — 200 gal boxCheapest reef salt with decent reef chemistry. Used to start.View on Amazon →Refractometer for saltwater + calibration fluidCalibrate with 35 ppt fluid monthly. Replaces hydrometers.View on Amazon →Rubbermaid Commercial Brute 14 gal containerStandard ATO reservoir. Food-grade plastic.View on Amazon →

Affiliate links. As an Amazon Associate, reefcalcs earns from qualifying purchases.

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Sources & references

1.
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/
2.
Global Seafood Alliance — Typical chemical characteristics of seawater
https://www.globalseafood.org/advocate/typical-chemical-characteristics-of-full-strength-seawater/

Last reviewed 2026-09-30