Net water, not tank rating. Use the Tank Volume calculator if you have not measured it.
What a water change actually removes, how many it takes to reach a target, and what it does to alkalinity.
| Change size | After 1 | After 4 | After 12 | Water used, 12 changes |
|---|---|---|---|---|
| 10 percent | 22.5 | 16.4 | 7.1 | 90 gal |
| 15 percent | 21.3 | 13.1 | 3.6 | 135 gal |
| 20 percent | 20.0 | 10.2 | 1.7 | 180 gal |
| 25 percent | 18.8 | 7.9 | 0.8 | 225 gal |
| 30 percent | 17.5 | 6.0 | 0.3 | 270 gal |
| 50 percent | 12.5 | 1.6 | 0.0 | 450 gal |
Dilution is exponential, which is why small frequent changes lose their grip. Twelve weekly ten percent changes leave 28 percent of the original behind. The tank also keeps producing nitrate the whole time, so real results land above this line unless export keeps pace.
final = 0.0 + (25.0 − 0.0) × (1 − 0.20)^1 = 20.00
A water change is a dilution. Replace a fraction f and the fraction left behind is 1 − f. Do it n times and what remains is (1 − f)ⁿ of the original.
When the new water is not clean, the formula gains a floor: final = c_new + (c_old − c_new)(1 − f)ⁿ. Concentrations decay toward whatever the replacement water holds, not toward zero. That matters for alkalinity and calcium, where fresh salt mix is often higher than the tank.
To find how many changes reach a target, invert it: n = ln((c_t − c_new)/(c_old − c_new)) ÷ ln(1 − f). If the target sits below the level in your new water, no number of changes will reach it.
The common disappointment: a 20 percent change removes 20 percent of the nitrate, not 20 ppm and not most of it. Going from 40 ppm to 10 ppm takes seven consecutive 20 percent changes with no new nitrate produced, and no tank produces none.
Water changes are for replacing trace elements and exporting the things nothing else removes. Using them as the main nitrate control is expensive and slow. Feed less, skim, run macroalgae, or dose carbon, and let water changes do what they are good at.
Match temperature and salinity to the tank before adding. Mix the new water at least overnight with a powerhead, and test it before it goes in rather than after.
Sources: Dieck, Reef Chemistry Calculator