How to lower nitrate in an aquarium
AquaLog guide · Published
In short: Change water: each 25% change removes about a quarter of the nitrate, so repeat until it reads below about 20–40 ppm. Keep it there by sizing your weekly change to how fast nitrate rises, feeding less, not overstocking and adding fast-growing plants. If your tap water already contains nitrate, water changes can't take the tank below that level.
What level should you aim for?
Many keepers aim for under 20 ppm, and most freshwater fish tolerate up to about 40 ppm, with sensitive species doing better at the low end. Nitrate has low toxicity, but high levels stress fish over time and feed algae. It is the end product of the nitrogen cycle: filter bacteria turn ammonia into nitrite and then nitrate, and in a typical aquarium only water changes and plants take it out again. See aquarium water parameters explained.
Check which unit your kit reports. If it gives nitrate-nitrogen (NO3-N) rather than nitrate (NO3), multiply the reading by about 4.4 to compare it with the targets here.
How much does a water change remove?
A water change removes the same share of nitrate as the share of water you replace, if the new water has none. So one 25% change takes out about a quarter, and a 50% change halves it. Starting from 80 ppm:
| Number of changes | Doing 25% changes | Doing 50% changes |
|---|---|---|
| 1 | 60 ppm | 40 ppm |
| 2 | 45 ppm | 20 ppm |
| 3 | 34 ppm | 10 ppm |
| 4 | 25 ppm | 5 ppm |
| 5 | 19 ppm | 2.5 ppm |
The table ignores the nitrate your fish add between changes, so real readings come out a little higher. If the tank has gone weeks without a change, spread the work over several days rather than doing one huge change; a sudden shift in water chemistry can shock fish. The water change calculator works out the change that gets you from your reading to a target, in one go or in smaller steps, and the aquarium volume calculator gives your tank's litres.
If your tap water contains nitrate, the sum becomes: new reading = tank reading × (1 − share changed) + tap reading × share changed. A 50% change on a tank at 80 ppm with tap water at 20 ppm leaves 80 × 0.5 + 20 × 0.5 = 50 ppm, not 40. This is the same sum the water change calculator uses.
Why does nitrate settle where it does?
If nitrate rises by about the same amount each week and you change the same share each week, the reading just before each change settles at a steady level: the weekly rise divided by the share you change, plus whatever your tap water contains. With nitrate-free tap water:
| Weekly rise | 10% weekly change | 25% weekly change | 50% weekly change |
|---|---|---|---|
| 5 ppm | 50 ppm | 20 ppm | 10 ppm |
| 10 ppm | 100 ppm | 40 ppm | 20 ppm |
| 20 ppm | 200 ppm | 80 ppm | 40 ppm |
To find your weekly rise, test straight after a water change and again a week later; the difference is the rise. If it is high, you can change more water each week, or cut the rise itself, which is less work in the long run. This is why 10–25% a week suits a lightly stocked tank while heavily stocked tanks need more; see how often to change aquarium water.
Does your tap water contain nitrate?
Test it. The World Health Organization's drinking-water guideline allows up to 50 mg/L of nitrate (the same as 50 ppm), and it notes that well water often carries more nitrate than surface water. Tap or borewell water near that level is already above an aquarium target before a single fish goes in.
If yours is high, reverse osmosis (RO) water removes most nitrate. Mixing RO water with tap water brings the level down while keeping some minerals. RO water on its own is very soft, so check the hardness your fish need on their page in the care guide.
How do plants help?
Nitrate is a plant nutrient. Fast-growing stem plants and floating plants take up nitrogen as they grow, and you remove it from the tank each time you trim them and throw the cuttings away. Slow growers take up less. Whatever the plants use is no longer available to algae.
Plants help most in a lightly stocked tank. They won't keep up with a tank full of large, messy fish, so treat them as a help, not a replacement for water changes.
How do feeding and stocking affect nitrate?
Apart from what your tap water brings, the nitrogen in an aquarium arrives as food. Fish digest it and release ammonia, which the filter turns into nitrate, and uneaten food rots into the same thing. Feeding less is the simplest way to slow the rise.
- Feed only what the fish finish in a couple of minutes, and remove leftovers.
- Vacuum the substrate at each water change to lift trapped waste before it breaks down.
- Rinse filter sponges in old tank water so they don't fill with sludge.
- Stock for adult size. Fancy goldfish need at least 150 litres (40 US gal) and produce a lot of waste, and a common pleco reaches 45 cm (18 in). Check every species in the care guide before you buy.
Nitrate creeps up slowly, so a single test tells you little. AquaLog charts every nitrate reading per tank alongside your logged water changes, which makes your weekly rise easy to read off.