How to lower nitrates in fish tank: 6 fixes that work
The fastest way to lower nitrates in a fish tank is a partial water change: swapping out 50% of the water roughly halves the reading, provided your tap water is low in nitrate. To keep it down, feed less, avoid overstocking, clean the filter and substrate, and add fast-growing plants. Most community tanks do well under about 40 ppm, and a very high reading should be brought down over several days rather than in one huge change.
Nitrate is the last product of the nitrogen cycle, and in a tank with no plants it only leaves when you remove water. So the answer to “how do I get it down?” is almost always the same: take water out, put less waste in, and let plants soak up some of the rest. Here are six fixes, in the order that usually works best.
Test the tank and the tap first
You can’t fix a number you haven’t measured. Use a liquid test kit, and shake the nitrate bottles as the instructions say, since the reagent settles and gives falsely low readings if you skip it.
Also test water straight from your tap. Some supplies contain nitrate, and if yours does, every water change adds some back. In that case the lowest reading you can reach is set by your tap, and you may need a different water source or a filter that removes nitrate, such as reverse osmosis, to go lower.
What counts as high? Opinions differ:
| Reading | What it means |
|---|---|
| 0–20 ppm | Good. Aim here if you keep sensitive fish, shrimp or a heavily planted tank that needs little else. |
| 20–40 ppm | Fine for most community tanks, but a sign to tighten up your routine. |
| 40–80 ppm | Higher than ideal. Start the fixes below and bring it down steadily. |
| 80 ppm and above | Too high. Reduce it over several days, then find the cause. |
Aquarium Co-Op suggests staying under 80–100 ppm at the very most, while TFH Magazine considers anything from 20–50 ppm too high and prefers 5–10 ppm. The University of Florida notes that nitrate is the least toxic nitrogen compound, but that newer research suggests it may be more harmful than once believed. A cautious target under 40 ppm sits comfortably between those views. Nitrate is far less dangerous than ammonia or nitrite, and fish often look fine until it gets very high, but long-term exposure still stresses them. For more on the whole set of numbers, see our water parameters guide.
1. Do a partial water change
This is the fastest and most reliable fix. A change of 30–50% with a gravel vacuum removes a large share of the nitrate in one go. If your tap water has next to no nitrate, the percentage you change is roughly the percentage you lower the reading by.
So going from 80 ppm to 40 ppm takes about a 50% change, and going from 60 to 40 takes about a third. You can check different tank sizes with the water change calculator.
Treat the new water with a dechlorinator and match its temperature to the tank, within a couple of degrees. Our water changes guide covers the full routine and how often to do it.
If nitrate is very high, go slowly. When a reading is well over 100 ppm, Aquarium Co-Op advises spreading the changes over several days instead of doing one huge swap, because large sudden shifts can shock fish. TFH Magazine suggests starting with about 15% and working up to 10–20% a day in a similar pattern until you reach a safe range. Fish that have lived in dirty water for months adapt to it, and a sudden jump to clean water can hurt them. A tank that has gone many months without a water change may also have a low pH, so make small daily changes there too. The Merck Veterinary Manual recommends this for old tank syndrome.
2. Feed less, and remove what the fish don’t eat
Overfeeding is the most common cause of high nitrate. Uneaten food rots, and fish turn the rest into waste, so every extra flake becomes nitrate.
- Give small portions that the fish finish in about two minutes.
- Siphon out leftover food and any dead fish, snails or plant leaves as soon as you spot them.
- Skip a day of feeding each week. Healthy adult fish cope fine.
The feeding guide explains how much to give for each type of fish.
3. Check your stocking
If nitrate climbs back to 40 ppm within a few days of a big water change, the tank may hold more fish than it can sensibly handle, or the fish may be bigger than when you bought them. Fewer or smaller fish is one of the most lasting fixes. Moving to a bigger tank works too, since it dilutes the same waste into more water.
Use the stocking calculator to see how your tank compares, and read how many fish per gallon for why the old inch-per-gallon rule misleads.
4. Keep the filter and substrate clean
Waste trapped in a clogged filter or deep in the gravel keeps breaking down and adding to the nitrate. Vacuum the gravel on each water change, and rinse sponges and other media in a bucket of old tank water, never straight under the tap, because chlorine can kill the filter bacteria. Don’t replace all the media at once. If you clean only part of it at a time, enough bacteria survive to keep the tank cycled. See how to clean a fish tank for the step-by-step routine.
5. Add fast-growing plants
Plants use nitrate as fertilizer, so a good planting helps lower it between water changes. Fast growers do the most. Aquarium Co-Op names water sprite and stem plants, and floating plants like Amazon frogbit are popular because their roots hang in the water and they get plenty of light. Slow growers such as anubias and java fern remove much less.
Some limits to know about:
- Plants slow the rise of nitrate, but rarely stop it. Keep doing water changes.
- A thriving planted tank needs some nitrate. If it reads 0 ppm, plants can run short and leaves may yellow or go see-through, so heavily planted tanks are usually kept with some nitrate, not none.
- Trim dead leaves quickly, since rotting plant matter adds nitrate.
- Check the rules where you live. Some floating plants are banned or invasive in certain areas.
Our easy plants guide lists forgiving choices for beginners.
6. Treat media and gadgets as extras
Nitrate-absorbing resins and denitrifying filters do exist. TFH Magazine describes them as useful add-ons that do not tackle the source of the problem. Building your own low-oxygen filter can take a lot of media and a long time to work, so it’s rarely practical in an ordinary home tank. Use these only after the first five fixes, and still test regularly.
What about nitrites?
Searches often pair nitrate with nitrite, but they are two separate problems. Nitrite is more dangerous, it should always read 0 ppm, and nitrate is what is left after bacteria have converted it. Any nitrite reading means the filter bacteria aren’t keeping up, often because the tank is new, the filter was cleaned too hard, or something has died.
If you see nitrite, do a large water change, hold back on feeding, and follow the steps in our ammonia guide, which apply to nitrite too. The nitrogen cycle guide explains why the tank went through the stages. Once nitrite is back to 0, return to the fixes above for the nitrate it produced.
Hidden sources of nitrate
If you’ve done everything and the reading stays high, look further:
- Tap water, as covered above.
- Fertilizers and root tabs that contain nitrogen.
- A dead fish or snail behind a rock or in the filter.
- A test kit that is past its date or wasn’t shaken properly. Try a fresh one.
A simple routine that keeps nitrate low
- Test nitrate once a week.
- Change 25–50% of the water weekly, adjusting up or down based on the reading.
- Feed small portions, and remove leftovers.
- Vacuum the gravel and rinse filter media gently in tank water.
- Trim dead plant leaves.
- Write down the readings. A pattern shows you whether your routine is enough.
Once the routine settles, you’ll know how fast your tank makes nitrate, and a single water change will rarely be an emergency again.
Frequently asked questions
How do I lower nitrates in a fish tank naturally?
Use fast-growing plants such as floating plants, water sprite or stem plants, feed only what the fish finish in a couple of minutes, and remove dead leaves and uneaten food. Plants take up nitrate as they grow, but they only slow the build-up, so you still need regular partial water changes.
How do I lower nitrates and nitrites in a fish tank?
They need different fixes. Nitrite should always read 0 ppm, and any reading means the filter bacteria are not keeping up, so do a large water change and see the ammonia and nitrogen cycle guides. Nitrate is what the bacteria produce afterwards, and it comes down with water changes, less waste and plants.
What nitrate level is too high for fish?
Sources differ, but a sensible target for most freshwater tanks is under about 40 ppm, with 5–20 ppm better for sensitive species. Fish often show no obvious signs until nitrate is very high, yet years of elevated levels can stress them and make disease more likely.
Does a water conditioner remove nitrate?
No. Normal dechlorinators and ammonia detoxifiers do not remove nitrate. Only removing water, plants, or specialist nitrate-removing media or systems lower it.
Why is my nitrate still high after a water change?
Check the tap water first, because some supplies carry nitrate of their own. Then look at overfeeding, too many fish, a clogged filter, rotting plant leaves, and nitrogen-containing fertilizers or root tabs.