How water changes affect fertilizer dosing
Water changes are half of any dosing plan. They set the ceiling your nutrients level off at, and changing less water means dosing less.
The steady-state formula
Each week you add an amount D and then remove a fraction f of the water. After a few weeks the level just before each change settles at about D ÷ f. Right after the change it drops to D ÷ f − D.
| Weekly change | Settles at (× weekly dose) |
|---|---|
| 25 % | 4× |
| 30 % | 3.3× |
| 50 % (EI) | 2× |
| 75 % | 1.33× |
So if you follow an EI dose sheet but change only 25 % a week, nitrate can climb to twice what the method intended.
Scaling doses to your water-change %
To keep the same ceiling with smaller changes, scale the weekly dose by your change fraction divided by the method's: switching from 50 % to 25 % means about half the dose. In the calculator, set the Water change / week field and watch the "builds to" numbers, or use the After a water change panel to top up just what a one-off change removed.
Try EI with 25 % changes and compare it with 50 %.
Do I re-dose after a water change?
With EI the next scheduled dose takes care of it, and many keepers dose macros right after the change. If you are rebuilding hardness with MgSO₄ or a GH product, add it after the change for the new water only.
Tap water matters
New water brings its own nitrate, phosphate, potassium, magnesium and sometimes copper. Check your water report and subtract what is already there.
Open the dosing calculatorMore guides
Sources
- Barr Report: EI and the 50 % weekly change (max 2× build-up)
- r/PlantedTank: adjusting EI to water-change size
- PPS-Pro: nutrient limits with 50 % weekly changes
Written in our own words from the sources above. Numbers are educational starting points; check product labels and your water.