Nitrate are the last nitrogen molecule of the nitrogen cycle and are directly available by the plants. They are beneficial for the plants but we must keep them below a certain limit as they can affect your fish health.


How Nitrate appear in the aquaponics system?

They are produced by bacteria from nitrite.


What action do they have on fish?

They can generate the following defects:


Strange behaviour (swim on the side, gap on the surface)

Poor feed conversion

Poor growth

Fast swimming

More disease

As you can see the impact of a high nitrate level on fish can be dramatic. For your fish welfare please keep them at an acceptable level.


What action do nitrate has on plants?

Nitrate are the main source of nutrients for the plants. Limited concentrations from 5 to 25ppm are perfect


What is an acceptable nitrate level for your fish?

Carps, catfish, goldfish and tilapia can leave in nitrate levels up to 120ppm but it is recommended to keep them 50ppm, with a target below 30ppm.

For trouts some sources report acceptance of levels above 500ppm but some scientific studies have shown effects below 100ppm! I would recommend to keep them in low concentration such as 30ppm as well.

How to balance nitrate in your aquaponics system?

If your nitrate are too high it may be because you don’t have enough plants to consume the production of your bacteria.

One of the options is to increase the number of plants in your system.

Another option is to use the aquaponics water to water your classic garden. It will lower the water level in your tanks and you will be able to top up the tank with new water which will dilute the nitrate and lower the concentration.

Welcome here! If you are new, you will probably be interested to discover Jonathan’s six steps to build and manage an Aquaponics system. Click here to access for free! Thanks and good reading 🙂

What are nitrates and what is an acceptable nitrate level in your aquaponics system? If you are new to this channel, my name is Jonathan and my aim is to help you to build your own aquaponics system and to manage it in the best way possible to grow some tasty, healthy, and sustainable food in your own backyard, thanks to aquaponics.


So today, we’re going to talk about nitrates in aquaponics. So nitrates are very interesting particles because they can be considered as a pollution and as well, they can be considered as a nutrient.


First, in aquaponics, nitrates are going to be very important because they’re going to be the source of food for the plants. But also if we got too much of them, at certain level, they’re going to start to be toxic for the fish.


So this is a molecule that we’re going to test in the water. They are part of the water parameters that we’re going to track and we’re going to make sure that we maintain it in a good level.


So basically, when you feed your fish, the fish are going to poo and they’re going to release some ammonia in the water. This ammonia is very toxic and it’s going to be transformed, thanks to some bacteria that we have in our grow-bed in aquaponics into nitrites. Nitrites are even worse than ammonia. But then thanks for there are some other bacteria that going to transform these nitrites into nitrates and the nitrates they are able to be absorbed by the plant.


So now we got a nitrate that is a fertilizer for the plant but very often, in an aquaponics system, we don’t have enough plants to absorb all the nitrates that are produced. So we need to test the water and to make sure that the nitrates are not building up in the setup because you test your water.


After a few weeks of running the setup, the nitrates are going to be quite low. After a few a months, if they increase a little bit of every week, after a while you know, they reach a certain concentration. And every species of fish has got some limits in the quantity of nitrate that they can absorb, they can take, without having some bad or unwanted effects.


What are those effects that they can have on fish? So a fish that is placed in the tank with a high concentration of nitrate, so nitrate levels that are above the maximum limits of the fish, the fish are going to start to change their behavior. You’re going find some that are going to swim very strangely. Some of them are going to start to swim on the surface because the nitrates, basically when the fish has got too nitrate it’s gong to stop the fish or it’s going to decrease the capacity of the fish to transport the oxygen inside his body. So the fish are going to start to come to the surface and to try to breathe the oxygen of the air.


If you see that and you know you got a high concentration of oxygen in your water, it may be because you have too much nitrates in the water. So it’s very important to check the nitrate levels.


If it’s for a long period of time, it can also have some anomaly in the shape of the fish. So you’re going to have some fish with very strange tail, very strange body, their body shape and it can be due to the nitrate level.


It will also affect your feed conversion ratio. So you know that when you feed your fish, you may put 1 kilo of fish food and from this 1 kilo of fish food which is dry, you may be able to produce let’s say 1 kilo of fish that is 80% moisture.


Now, if you have high concentration of nitrates above the limit of the fish, while with 1 kilo of fish food, you may be able to produce only 800 grams of fish. Basically, you’re going to decrease the efficiency of the system, the efficiency of the ecosystem to produce some fish.


So it has a significant impact on the productivity of the system. It can also affect the growth rate of your fish. So if the fish are able to reach the plate size in 1 year, maybe that if you got a high concentration of nitrates, it may take them 2 years to reach the plate size. So it’s another effect of nitrates, their growth.


And finally, if the fish are stressed due to high nitrate level, they’re going to be way more likely to get any disease because when a fish is stressed, the barrier against the disease are falling down, and that’s where the fish is very, very highly exposed to disease.


Finally, if the level is very, very high the fish can simply die. So you see the importance to maintain low nitrate concentration. So when we talk about nitrogen levels, we are talking about ammonia, nitrite, and nitrates. Ammonia and nitrite are very toxic but nitrate the concentration that the fish can take is way higher. Now, this concentration is specific to every species.


So in aquaculture, the fish farmers sometimes they keep their fish at very high level of nitrate if they add pure oxygen and those types of things. But in aquaponics, we don’t want that at all.


For trout, if you are above 100 ppm, you can start to see some problems. So it is recommended to keep it below 30 ppm. So 30 ppm is not much. Most of the plants they enjoy a nitrate level that is around 30 ppm. From 10-30 is perfect for the plant. But for the fish, if you go above 30 ppm, then you can start to have some little problems. So most of the time, up to 100 ppm, you can’t really see any real difference but after 100 ppm, your fish are not going to be at ease and they may start to stress. And that’s where you’re going to have some troubles in your aquaponics system. You may have some fish that just get some disease and die.


So really, check your nitrate level. Now, to check the nitrate level, it’s very simple. You get a water test kit. So it’s the same as for the aquarium. It’s just some little bottles. You put some reactive inside and you can read the exact level of nitrate that you have in your system. It’s very simple to do. I recommend to do it at the beginning once every week, and then you can do it a bit less often once you got some experience and you are used to your aquaponics system.


But please, don’t leave the nitrate level to go to the roof because it’s going to be very painful for your fish. They’re not going to be happy. So check it often and if you see that the nitrate level is too high, there is a very simple solution to decrease it. It’s to use this water to water your classic garden. If you use some water from your aquaponics system to water your classic garden, you’re going to decrease the water level in your aquaponics system and you will have to add new water, and this new water is going to be zero in nitrates.


Rain water or tap water, whatever you use, it’s going to be very low in nitrate so it’s going to dilute the nitrate level of your aquaponics system.


So now if you are new to aquaponics and you need to know the different ratio that you respect, the different limits of the system when you build your system but also when you maintain it, I highly recommend you to get the free training in the description of this video just below. It’s a free training, completely free, where you get all the critical information that you require, that you need to start aquaponics on good basis but also to get some knowledge to manage the aquaponics system in the best way possible.


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