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 values
Replacement-water values
Pure dilution: new = old × (1 − x) + replacement × x. Assumes no biological draw-down during the change.
Gear for this
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
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
7–8 dKH spec. The go-to for calcium-reactor and Triton tanks.
View on Amazon →
Instant Ocean Reef Crystals — 200 gal box
Cheapest reef salt with decent reef chemistry. Used to start.
View on Amazon →
Refractometer for saltwater + calibration fluid
Calibrate with 35 ppt fluid monthly. Replaces hydrometers.
View on Amazon →
Rubbermaid Commercial Brute 14 gal container
Standard ATO reservoir. Food-grade plastic.
View on Amazon →
Affiliate links. As an Amazon Associate, reefcalcs earns from qualifying purchases.
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<iframe
src="https://reefcalcs.com/embed/water-change/"
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style="border: 1px solid #1C2D34; border-radius: 8px; max-width: 720px;"
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title="ReefCalcs Water Change Calculator"
></iframe>Sources & references
- 01Randy 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/
- 02Global Seafood Alliance — Typical chemical characteristics of seawaterhttps://www.globalseafood.org/advocate/typical-chemical-characteristics-of-full-strength-seawater/
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