Nitrates are, for most aquariums, the last stage of the Nitrogen cycle. A healthy aquarium usually builds nitrates up quite quickly and is one of the reasons that we conduct regular maintenance and water changes.
So aquarium nitrates are certainly worth talking about.
It turns out though, that nitrate is not quite as toxic as many aquarists believe it to be and I have seen a lot of confusion recently on how to measure it. There has also been a lot of debate on the accuracy of some testing kits. So hopefully we will set things straight here.
Toxicity of Nitrates
There are millions (possibly exaggerated) of websites and articles that will tell you that nitrates are toxic from as low as 80ppm (nitrate-nitrogen), this is the figure that many aquarists believe to be the toxic level of nitrate in the aquarium.
This isn’t strictly true, but it is a good figure to keep in mind and a good limit to have for your nitrate levels which should still, ideally, be kept as low as possible.
Research has shown that actually, nitrates are not toxic for most species of fish until they hit 300ppm+ (nitrate-nitrogen), even higher for some species such as Goldfish who can live happily at levels above 4400ppm (nitrate-nitrogen). Though fry and eggs are noted in some studies to be more sensitive and can be more severely affected by levels above 80ppm (nitrate).
So what effect do nitrates have on our fish?
As we have discussed, nitrate is toxic and in high enough concentrations it can be lethal to fish. The lethal level though is significantly higher than many of us think it is and at levels lower than that, such as the highest levels that testing kits read, the effects are pretty limited.
One effect that high levels of nitrate have is that, if prolonged, the nitrate can reduce the life expectancy of our fish.
At around 130ppm (nitrate-nitrogen) the life expectancy of a fish could be reduced by around 0.05%. This was a result of a single study.
Many of us would have a heart attack at seeing a figure of 130ppm (nitrate-nitrogen) on our test, on an API master test kit this figure would show as 575.9ppm – real panic time! Although, the tests do not actually read that high. I’ll cover why these readings are different shortly.
Some studies show a reduction in the growth rate of fish fry when nitrate is at a concentration of 1000ppm+ (nitrate-nitrogen). This means that nitrate can affect the growth rate of fish, but only at higher concentrations than you may think.
So, the effects of high nitrate levels are fairly limited and are not an issue until they are in pretty high concentrations. However, it is still good practice to keep nitrate much lower than these figures.
Aquarium Nitrate Testing
There are a whole host of aquarium test kits available for testing various aquarium parameters, including aquarium nitrate.
Two of the more popular kits are made by API and NT labs. Recently I have seen a lot of talk on how inaccurate the API kit is and that NT labs is better, there is a reason for this.
If you tested your aquarium nitrate levels with the API master test kit and got a reading of 80ppm, the same test with an NT labs kit would show around 20ppm (slightly below).
This has led many to assume that the API kit is inaccurate, probably because it makes them feel much better to see a figure around 20ppm than it does to see 80.
The truth is that both of these kits are showing the same result and are both accurate when used correctly, in my experience.
So why are they different?
You may have noticed that the API test kit chart doesn’t turn to a red (bad) colour until 80ppm but the NT labs chart has a skull and crossbones (bad) at 20ppm.
The reason that these two kits give different results and tell you that bad things will happen at different levels is that they measure different things.
The API kit measures nitrate, the thing we are actually trying to measure.
The NT labs kit measures Nitrate-Nitrogen (the figures given above).
Both these measurements are accurate and are recognised ways to measure aquarium nitrate but to equate nitrate-nitrogen to nitrate the result given needs to be multiplied by 4.43.
This means that a result of 20ppm on an NT labs nitrate test is the same as a result of 88.6ppm on an API nitrate test.
Hopefully this explains the difference in the results given by these two popular testing kits. Most testing kits that I have used measure nitrate-nitrogen.
Aquarium Nitrate removal
Our filtration breaks down nitrogenous waste in our aquarium, ammonia is converted to nitrite and nitrite into nitrate. But most filters will not break nitrate down.
This is because the breakdown of nitrate requires anaerobic bacteria, which is very difficult to culture in an aquarium.
Whilst nitrate is not all that toxic to most adult fish, it is still best to keep it as low as possible, which is why I think that 80ppm nitrate, or 20ppm nitrate-nitrogen is a good level to hold as the upper limit for your tank. Levels above this can have adverse effects on sensitive fish, juvenile fish and eggs.
So how are nitrates removed?
As nitrates build-up, we need to remove them from the aquarium. There are several ways to do this, one of the most common and easiest ways is by water changes.
Water changes allow us to quickly and effectively dilute the nitrates in our aquarium. If your nitrate level is 80ppm, then a 50% water change should reduce that to 40ppm. An 80% water change should reduce it to 16ppm, and so on.
Another great way to keep nitrates down is to use live plants. Live plants use ammonia, nitrite and nitrate as food. So planting your aquarium can help to keep nitrate levels down and reduce the requirement for water changes.
Some filter media claims to be able to allow the growth of anaerobic bacteria and remove nitrate from the aquarium. A couple of well-known media that claim this are Biohome and Seachem Matrix.
Biohome media is fairly expensive and is a popular choice amongst aquarists. However, it was recently tested by Kev at Kaveman aquatics which did not show positive results (video below). I am currently in the process of testing Seachem Matrix in a very similar way, I will post the results once I have fully tested it.
Some water conditioners will help to reduce nitrate. Quantum water primer is my conditioner of choice and it will reduce aquarium nitrate levels, but I would not recommend it as a substitute for good aquarium maintenance.
So, it would seem that the best ways to remove nitrate are with live plants and regular maintenance.
So, it would seem that nitrate is not as toxic as it is often thought to be, but it can still affect the lives of our fish.
My recommendation is to keep 80ppm nitrate (API test) or 20ppm nitrate-nitrogen (NT labs test) as your upper limits, but there is absolutely no reason to panic if your nitrate levels creep over this. It is very unlikely that your fish will immediately drop dead and it is also unlikely that any harm will come to them at all providing that the nitrate is reduced in a timely manner.
If you do find your nitrates rising above 80, do a water change. Nitrate is often the factor that sets our maintenance schedule and should be checked regularly to ensure that our water changes are well-timed and large enough to keep the nitrate concentration as low as possible.