Aquarium Water Change Calculator
Fill in the values and the answer updates as you type.
How the Aquarium Water Change Calculator works
After replacing a fraction x of the water, nitrate = current × (1 − x) + tap × x. Solving for x gives x = (current − target) ÷ (current − tap). The result assumes the new water mixes instantly and that nothing else adds nitrate during the change. Big changes can shock fish through shifts in temperature and pH, so split a large change into two or three smaller ones over a few days and match the new water to the tank. Example: Lowering a 200 liter tank from 60 ppm to 20 ppm with 5 ppm tap water needs 72.73% replaced, or 145.5 liters.
Inputs and results
The Aquarium Water Change Calculator takes 4 inputs and gives 4 results.
What you enter
- Water volume (liters) — starts at 200.
- Current nitrate (ppm) — starts at 60.
- Target nitrate (ppm) — starts at 20.
- Nitrate in tap water (ppm) — starts at 5.
What you get
- Water to replace (%)
- Water to replace (liters)
- Water to replace (US gallons)
- Nitrate after the change (ppm)
Example: Aquarium Water Change Calculator with the starting values
With water volume 200 liters, current nitrate 60 ppm, target nitrate 20 ppm and nitrate in tap water 5 ppm, the calculator returns:
| Water to replace (%) | 72.73% |
|---|---|
| Water to replace (liters) | 145.5 |
| Water to replace (US gallons) | 38.4 |
| Nitrate after the change (ppm) | 20 |
Change any value above and the results update straight away.
Aquarium Water Change Calculator FAQ
How do I use the Aquarium Water Change Calculator?
Enter water volume, current nitrate, target nitrate and nitrate in tap water in the fields, then read water to replace (%), water to replace (liters), water to replace (us gallons) and nitrate after the change (ppm) in the result panel. The answer updates as you type, and you can press Calculate or Enter to refresh it. Use “Copy link” to share your exact numbers with someone else.
What does the Aquarium Water Change Calculator calculate?
How much water to replace to bring nitrate down to your target. It returns water to replace (%), water to replace (liters), water to replace (us gallons) and nitrate after the change (ppm) from the 4 values you provide.
What values does the Aquarium Water Change Calculator start with?
It opens with water volume 200 liters, current nitrate 60 ppm, target nitrate 20 ppm and nitrate in tap water 5 ppm so you can see a working result straight away. Replace them with your own numbers, or press Reset to return to these starting values.
Is the Aquarium Water Change Calculator free and private?
Yes. It is free, needs no account, and runs in your browser, so nothing you type is sent to or stored on our servers. See the Privacy Policy for details.
Can I rely on the Aquarium Water Change Calculator for medical or health decisions?
Treat the result as a well-rounded estimate. Real situations often have details a general calculator cannot know, so confirm medical or health decisions with a qualified professional. See the Disclaimer for more.