Safe Nitrate Levels for Aquarium Fish: Why Sources Say 20, 40 or 80 ppm

Check your unit first

Most of the confusion starts here. Nitrate shows up in two forms in the literature. Research papers usually give nitrate-nitrogen (NO3-N), which counts only the nitrogen. Hobby kits like the API test read full nitrate (NO3). Merck notes that some kits report nitrate-nitrogen only and that you multiply by 4.4 to get nitrate 1. Aquarium Science uses the same factor, and says a reading of 10 on a scientist’s kit equals about 44 on an API-style kit 3.

That’s our multiplication by 4.4. Read your kit’s label. If it says “as N”, multiply. Our water change guide makes the same point about kits. In the rest of this page, “ppm” means nitrate as the API-style kit reads it, and we say so when a source uses nitrogen.

What nitrate does, and how toxic it is

Nitrate is the end of the nitrogen cycle: ammonia becomes nitrite, nitrite becomes nitrate, and nothing in a normal tank removes it for you. Merck calls it less toxic than ammonia or nitrite 1.

Less toxic doesn’t mean harmless. Camargo and colleagues reviewed the research in 2005. The abstract says nitrate’s main toxic action is converting oxygen-carrying pigments into forms that can’t carry oxygen. Toxicity rises with concentration and exposure time, and may fall with larger body size, higher salinity and adaptation. Freshwater animals appear to be more sensitive than marine ones 5.

Why the sources disagree

The strictest numbers come from toxicology on animals that aren’t aquarium fish. Camargo’s abstract says 10 mg/L nitrate-nitrogen (44 ppm) can adversely affect freshwater invertebrates, fishes and amphibians, at least over long exposures. It recommends safe levels below that, and says a maximum of 2 mg/L nitrate-nitrogen (about 9 ppm) would be appropriate for the most sensitive freshwater species 5. The species it names are aquatic insects and amphipods, fish in the trout and salmon group, and several frogs and a toad. None is a tropical aquarium fish, so the numbers don’t map neatly onto a home tank. That mapping is our reading; the abstract doesn’t discuss aquariums.

The loosest numbers come from aquarium and lab fish. A 2017 study kept juvenile zebrafish for 28 days at 100, 200 and 400 mg/L nitrate-nitrogen. Nobody died up to 200 (about 880 ppm). At 400 (about 1,760 ppm), 47% died. Growth and tissue damage got worse as nitrate went up, and the authors recommended staying below 100 mg/L nitrate-nitrogen (about 440 ppm) for juveniles 6. Aquarium Science builds on studies like this one and lists chronic limits of 440 ppm and up, depending on species 3.

So there are two defensible readings. One says adult tropical fish shrug off tens of ppm and the old 20 ppm rule is overcautious. The other says sensitive animals show effects at 44 ppm and below. Aquarium Science takes the first view. It says it found no evidence that anything under 80 ppm on an API-style kit even affects adult fish, and that the over-20 rule of thumb is unsupported 3. Merck, the more conservative of the two, gives under 20 mg/L as the freshwater reference range and doesn’t say which unit it means 2. We read the evidence this way. Adult hardy fish probably tolerate far more than 40 ppm. The number that matters in practice is the lowest-tolerance animal in your tank.

What we’d aim for

This is our judgment from the figures above, not a published rule.

• Community tank with ordinary fish. Change water before nitrate passes 40 ppm. That’s where Aquarium Co-Op puts the line 8, and it’s about the same as Camargo’s long-term effect level of 44 ppm. Below that you’re in comfortable territory. If you drift to 50 or 60 between changes now and then, no source we read says that’s an emergency.
• Shrimp, axolotls and young fry. Aim lower. Aquarium Science says 40 to 80 ppm can affect shrimp and axolotls and some sensitive fry 3. The new invertebrate data in Camargo’s review covers two amphipods and a caddisfly, and we found no study on aquarium shrimp, so this is a cautious call. Our axolotl guide lists targets from under 10 to under 40 ppm.
• Goldfish. Aquarium Science says pond-bred fish handle more nitrate than fish from mountain streams, and that common goldfish can withstand very high levels 3. Our goldfish guide covers the rest of their care.

The cost of a lower target is more water. Aquarium Science estimates that holding 20 ppm takes about four times as many water changes as holding 80, if your tap has no nitrate 3. The right number is the lowest one you’ll actually stick to.

Test it properly

A reading is only as good as the test behind it.

Aquarium Science says the nitrate reagents are prone to aging and shaking errors. A bottle open for more than six months at room temperature can give wrong readings, and the directions on shaking have to be followed closely 3. It also says that with the API kit you should judge by the orange of the tube, not how dark it is, and gives orange as 40 ppm 3 4. Its practical target is “orange-red but not red-red” 3.

Test your tap water too. The EPA sets a legal limit of 10 mg/L nitrate-nitrogen for drinking water, about 44 ppm on an API-style kit 7. If your tap reads 20 ppm, your tank can never read lower than that, and no amount of water changing will get it there. That’s our arithmetic. The steady-state example in the water change guide shows how the floor works.

If you read zero. Aquarium Science lists five reasons: a lightly stocked and lightly fed tank, plants absorbing it, frequent water changes, shaking the test too little, and food under 40% protein, which can tie nitrate up in the brown gunk in the tank 3. A zero in a tank that’s cycling is a different case and the answer is in the cycling guide. Aquarium Co-Op’s definition of a finished fish-in cycle includes nitrate above zero 8.

How to bring nitrate down

1. Water changes. They’re the only thing that removes nitrate for sure. How often depends on stocking and the number you picked, which our water change guide works through.
2. Plants. Aquarium Co-Op says aquatic plants use nitrogen waste even more effectively than bacteria, citing microbiologist Diana Walstad 8. A FishLore member added pothos to a tank that ran above 40 ppm for exactly that reason 9.
3. Feed less or stock less. Aquarium Science links low readings to a lightly stocked, lightly fed tank 3. More fish and more food mean more nitrate. There’s no way around that.
4. Keep detritus out. Old food and waste break down into the same nitrogen compounds. Vacuum when you change water.

What hobbyists report

On a FishLore thread about target nitrate, one member said two of their tanks sat around 20 ppm and a third ran at 40 and higher, with no ill effects they could see in the fish. They were trying pothos in that tank 9. A second member keeps a heavily stocked 60 gallon with growing cichlids and changes half the water when nitrate reaches 10 ppm, which works out to anywhere from two changes a week to three every two weeks. Their advice to the original poster, who wondered whether staying under 5 ppm in a planted tank was overkill: let the test tell you how much to change 9. Aquarium Science quotes a keeper who moved to monthly changes and only tried to stay below roughly 80 to 100 ppm, and reports corydoras breeding and zebra danios raising fry alongside angelfish 3. Same hobby, very different habits, and none of the reports mention fish lost to nitrate.

FAQ

Is 40 ppm nitrate bad? Not in a tank of hardy fish, according to most of the sources here. Aquarium Co-Op treats it as the point to change water 8. Aquarium Science says most hobbyists are comfortable between 40 and 80 3. Camargo’s long-term effect level for sensitive animals is about 44 ppm 5, and Aquarium Science says shrimp and axolotls can be affected from 40 3, so we’d aim lower for them.

Can nitrate kill fish? At the right dose, yes, but the doses are high. In the zebrafish study, deaths started at 400 mg/L nitrate-nitrogen, about 1,760 ppm 6. Most tanks never come close. The risk in a neglected tank is more often the ammonia and nitrite that come with it.

Is zero nitrate a problem? Not by itself. Plants, light stocking and frequent water changes can all keep it near zero 3. In a new tank, a zero with ammonia or nitrite still showing means the cycle isn’t done.

How often should I change water for nitrate? It depends on your stocking and your target. Our water change guide has starting schedules and the arithmetic.

Do plants really remove nitrate? Aquarium Co-Op says they take up nitrogen waste more effectively than bacteria do 8. How much a given tank drops depends on how many plants it has and how fast they grow.