Percent and gallons of water to change in a fish tank, worked out from how fast nitrate rises between two tests, so it stays under your target on the schedule you keep.
Aquarium Water Change Calculator - Percent from Nitrate
Percent and gallons of water to change in a fish tank, worked out from how fast nitrate rises between two tests, so it stays under your target on the schedule you keep.
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Aquarium water change calculator, sized by your own nitrate test
Rules like "25% a week" ignore the one number that says how fast your tank fouls its water: the nitrate rise between two tests. This aquarium water change calculator takes that rise, your tank volume and a target, and returns the percent and the gallons to change on your schedule. A 20 gal tank going from 5 to 12 ppm in a week needs 35%, or 7.0 gal, every week to stay at or under 20 ppm.
A fixed 25% every week
- Same percent whatever the fish load
- Nitrate creeps up to 28.0 ppm before each change in the example tank
- Tap water nitrate never enters the sum
A percent from two tests
- 35% holds the same tank at 20.0 ppm
- Shows what a smaller, more frequent change does
- Tells you when the tap water is the real problem
Two tests and a tank size
- Volume unit and water volume - US gallons or liters of water actually in the tank, not the size on the label.
- Nitrate right after the last change - in ppm (mg/L) from a liquid test kit.
- Nitrate today - before any change, with the same kit.
- Days between the readings - turns the difference into a rise per day.
- Days between water changes - optional, empty means every 7 days.
- Target nitrate - optional, empty means 20 ppm.
- Tap water nitrate - optional, the reading of the water you add.
- Planned change - optional percent, to see where your current habit leaves the tank.
- Read the percent, the gallons and the schedule table.
The dilution arithmetic behind the percent
fraction to change = rise between changes / (target - tap)
Change a fraction of the water on a steady schedule and nitrate settles into a sawtooth: it climbs by the same amount between changes and drops by the same amount at each one. The top of that sawtooth is the number that matters, and it lands where the formula says.
The calculator rounds the percent up to one decimal, so following it keeps the tank at or just under the target rather than just over.
What each schedule costs in water and buys in nitrate
Nitrate just before each change in the example tank (rise 1 ppm a day, tap 0), and the water it takes per week. The target rows are the ones at or under 20 ppm.
| Schedule | Per change | Nitrate before change | Water per week |
|---|---|---|---|
| 10% every 7 days | 2.0 gal | 70.0 ppm | 2.0 gal |
| 25% every 7 days | 5.0 gal | 28.0 ppm | 5.0 gal |
| 35% every 7 days | 7.0 gal | 20.0 ppm | 7.0 gal |
| 50% every 7 days | 10.0 gal | 14.0 ppm | 10.0 gal |
| 15% every 3 days | 3.0 gal | 20.0 ppm | 7.0 gal |
| Both schedules that hit 20 ppm | 7.0 or 3.0 gal | 20.0 ppm | 7.0 gal |
The last two rows are the surprise. Holding the same target takes the same water per week however you split it, because the fraction needed shrinks exactly as fast as the changes multiply. What frequent changes do save is the swing: nitrate moves by 3 ppm between changes instead of 7.
Nitrate as N or as NO3: the two scales on test kits
Drinking water rules count nitrate as nitrogen; the US EPA limit is 10 mg/L measured that way. Many aquarium kits read nitrate as the whole NO3 ion. To compare, multiply nitrate-N by 4.4268, the ratio of the two molar masses.
| Nitrate as N (mg/L) | Nitrate as NO3 (ppm) |
|---|---|
| 1 | 4.4 |
| 2 | 8.9 |
| 5 | 22.1 |
| 10 (EPA limit for tap water) | 44.3 |
Tap water that meets the legal limit can still carry more nitrate than a 20 ppm target allows, and then no schedule will get there. The calculator refuses that case and says why.
Water change questions, short answers
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