High nitrates in fish tank: safe levels and how to lower them

High nitrates in a fish tank are lowered mainly by water changes, feeding less and adding plants. For most freshwater fish, keep nitrate under about 20–40 ppm (parts per million, the same as mg/L); aim for under 20 ppm for shrimp, wild-caught fish, discus and other sensitive species.

Nitrate builds up slowly in every tank, so a high reading is rarely an emergency. But fish living with it for months grow more slowly, get sick more often and live shorter lives. Very high levels (often well over 100 ppm) can kill. Test your tap water too: it may already contain nitrate.

How to identify it

You can't see or smell nitrate. The only way to know is to test. Signs that might point to high nitrate include:

Fish that are dull, listless or slow-growing.
Frequent disease outbreaks with no obvious cause.
Shrimp dying, especially during moults.
Persistent algae, especially green spot, hair or green water.
Fish showing stress after a long gap between water changes.

What nitrate is

Fish waste and rotting food release ammonia. Bacteria in your filter turn ammonia into nitrite, then nitrite into nitrate. This is the nitrogen cycle (see cycling). Ammonia and nitrite are very toxic; nitrate is much less so, but nothing in a normal filter removes it. It keeps building up until you take it out with water changes or plants use it.

Reading your test

Most hobby test kits read in ppm of nitrate (NO3). Some lab and scientific kits read "nitrate-nitrogen" (NO3-N). Multiply NO3-N by about 4.4 to get nitrate. So 10 ppm NO3-N is about 44 ppm nitrate. Check your kit's instructions, and shake the second bottle hard: liquid nitrate kits under-read if you don't.

When it's a problem

Nitrate (ppm NO3)	What it means
0–20	Good for nearly all fish, shrimp and plants.
20–40	Acceptable for most hardy community fish. Time to do a water change.
40–80	Too high. Long-term stress and more disease. Lower it over the next week.
Over 80–100	Serious. Fish may be lethargic and sick. Lower it steadily over several days.

Experts don't fully agree on safe levels. Older advice said nitrate was harmless up to 200 ppm or more; newer research shows long-term harm at much lower levels, particularly to young fish, invertebrates and amphibians. A widely cited 2005 review suggested very low limits to protect the most sensitive wild freshwater species. For aquarium fish, under 20–40 ppm is a sensible, achievable target.

What to do right now

Test the tank for nitrate, ammonia and nitrite.
Test your tap water for nitrate. If it reads 20 ppm or more, water changes alone won't get you lower; see below.
If ammonia or nitrite is above zero, deal with that first: see ammonia and nitrite poisoning.
Do a 25–30% water change with dechlorinated water at the same temperature.
Cut feeding to what is eaten in about two minutes, once a day.
Remove waste: vacuum the gravel, remove dead leaves and any uneaten food.

What to do next

Bring it down gradually

If nitrate is very high (over about 80 ppm), don't do one huge water change. Fish that have slowly adapted to poor water can be shocked by a sudden change in nitrate, pH and hardness all at once. Instead, change 25–30% every day or two until you reach your target. Each 25% change removes about a quarter of the nitrate.

Find out why it's high

Overfeeding: the most common cause. Food that isn't eaten all becomes nitrate. See overfeeding.
Overstocking: more fish, more waste. Large fish such as plecos, goldfish and cichlids produce a lot.
Too few water changes: most tanks need 20–30% weekly. Heavily stocked tanks need more.
Dirty filter or substrate: trapped sludge keeps releasing waste. Rinse media in old tank water and vacuum gravel.
Tap water: in farming areas, tap or well water can carry nitrate. Use a test to find out.

If your tap water already has nitrate

Mix tap water with reverse-osmosis (RO) water or clean rainwater collected away from roofs with copper or tar. RO water has no minerals, so you must remineralise it or mix it with tap water.
Use more plants and fewer, smaller fish.
Consider a nitrate-removing resin or filter media made for aquariums.

Plants

Fast-growing plants use nitrate as food. Floating plants such as frogbit, salvinia and water lettuce, and stem plants such as hornwort and water sprite, are the best. Trim and remove the growth regularly, because that is how the nitrate leaves the tank. Pothos or other houseplants with their roots in the water also help.

Cautions

Shrimp and snails are more sensitive to nitrate than most fish, and also to sudden changes; keep changes moderate and steady. If fish are already sick and you need to medicate, fix the water first, remove carbon while dosing, and take care with plecos, loaches, scaleless catfish and invertebrates.

Products that help

Liquid nitrate test kit: more accurate than strips. Test weekly and your tap water once.
Gravel vacuum: removes waste before it turns into nitrate.
Water conditioner: makes regular water changes safe.
Fast-growing and floating plants: soak up nitrate naturally.
Nitrate-removing resin or media: useful when your tap water is high in nitrate; follow recharge instructions.
RO unit or bought RO water: for very high nitrate tap water, mixed with tap or remineralised.

Keep on hand: a nitrate test, conditioner and a long hose or water-change pump so changes are easy enough that you actually do them.

How to prevent it

Change 20–30% of the water every week.
Feed small amounts and skip a day each week.
Stock lightly, and plan for adult fish sizes.
Keep live plants.
Clean the filter and substrate regularly.
Test nitrate weekly so you notice the trend.

Why it happened

Nitrate is the end product of your filter doing its job. In a closed tank with few water changes, it just keeps building. A high reading means waste is coming in faster than you are taking it out.

When to get help

Get help if nitrate stays high despite weekly changes, if your tap water is very high, or if fish are sick or dying. A local fish shop can test your water and suggest a water source. For a well, your local health authority can test drinking water nitrate.

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