Aquarium water change calculator
Enter your tank volume and nitrate reading. The calculator gives the share of water to replace to reach a target nitrate, counting the nitrate that is already in your tap water.
Change plan
One change of 50% gets to the target. Retest a day later.
| Change | Replace | Amount | Nitrate after |
|---|---|---|---|
| 1 | 50% | 14.5 gal | 20 mg/L |
What's next
I'm thinking about a log that keeps each nitrate reading next to the water change that followed, so a chart shows how fast nitrate climbs between changes and whether your schedule holds it under your target. It isn't built yet. The button below only adds one to a counter, so I can see if enough people want it.
Thanks, counted.
How it works
Replacing a share f of the water leaves nitrate at N × (1 − f) + tap × f. The calculator solves that for f. With clean tap water it is (now − target) ÷ now: from 40 to 20 mg/L takes a 50% change.
Changes over 50% are split into equal smaller ones, so the plan shows the fewest changes of at most 50% each. Retest between them, since feeding and plants move the number too.
Tap water that already has nitrate raises the amount you need. From 40 to 20 mg/L with 10 mg/L in the tap takes 66.7% instead of 50%.
This is the nitrate in the water only. Plants and filters also change it over time, so test again a day after the change.
For what repeated changes do to a high reading, see nitrate too high. To work out how often to change water from your weekly nitrate rise, see how often to change aquarium water.
Adding tap water? The dechlorinator dose calculator turns the volume into mL of Seachem Prime or API conditioner.
Dosing fertilizer in a planted tank? The fertilizer calculator converts grams to mg/L.
Planning bucket trips? See water change by tank size. Keeping shrimp? Their changes are smaller, see shrimp tank water change.
Not sure how much water is in the tank? Use the tank volume calculator. Gravel, sand and rock take up room, so the real volume is lower than the tank's label.
A worked example: 40 gallons at 40 ppm
A 40 gallon tank reads 40 ppm nitrate. The tap water has 10 ppm and the target is 20 ppm. Enter those four numbers and the calculator gives:
- One change would be (40 − 20) ÷ (40 − 10) = 66.7%, or 26.7 gallons.
- Because that is over 50%, the plan splits it into 2 changes of 42.3%, 16.9 gallons each.
- After the first change the tank reads 27.3 ppm. After the second it reads 20.0 ppm.
Two changes use 33.8 gallons in total, more than the single change of 26.7, because the second change replaces some water that is already fresh. In exchange, no single change is bigger than 42.3%. With clean tap water at 0 ppm the same tank needs one 50% change of 20 gallons. A routine 25% change with 10 ppm tap water takes 40 ppm to 32.5 ppm, since 40 × 0.75 + 10 × 0.25 = 32.5.
What this does and does not do
- It does the arithmetic of dilution. Your tap water sets the floor: with 15 ppm in the tap, no number of changes brings the tank below 15 ppm.
- It cannot tell you what is safe for your fish or plants. Targets vary: Aquasabi suggests 10–25 mg/L for planted tanks. Check what your fish and plants need.
- It stores nothing about you. The numbers you type are not saved, and the copy link only puts them in the address of the page. Page visits and calculator uses are counted without cookies or IP addresses.
- Liters and US gallons both work. One US gallon is 3.785 liters.
Questions people ask
- Why not change all of it at once?
- The plan keeps each change at 50% or less. My suggestion: spread very high readings over several changes. Match the new water's temperature to the tank.
- When do I retest?
- A day after a change. Feeding, plants and the filter keep moving the number, so the reading is a better guide than the plan.
- The calculator says the target is unreachable. Why?
- Your tap water has as much nitrate as the target, or more. Raise the target above the tap reading, or use water with less nitrate.
- What volume do I enter?
- The water in the tank, which is less than the label on the glass says. Gravel, sand and rock take up room. The tank volume calculator subtracts that.