Nitrate is the last nitrogen molecule of the nitrogen cycle. In aquaponics we aim to transform all nitrogen molecules into nitrate. Because it is less toxic for the fish and directly assimilable by the plants. Although, we must keep them below a certain limit. Indeed, at high concentration, nitrate can affect your fish health!
In this article we will see what are nitrate and what level to aim for in aquaponics. If you are looking for a quick answer, this table will help you. Please note, the nitrate values given here are average and can be adjusted for each species.
| Nitrate level | Plan of action | |||
| above 80 ppm (high) | use aquaponics water to water classic garden | Harvest some fish | increase veggies area | Clean growbed (remove extra organic matters) |
| below 5 ppm (low) | add more fish (if possible) | add fertilizer | ||
How do nitrate appear in the aquaponics system?
Fish food
The fish pellet we use to feed our aquaponics fish are rich in protein. Proteins contain nitrogen atoms. when digested by the fish, some end up into the fish waste under the form of ammonia. Ammonia is very toxic for the fish. But fortunately, some bacteria (like Nitrosomonas sp.) will transform ammonia into nitrite. Nitrite is also very toxic for the fish. However, some bacteria (like Nitrobacter sp.) consume the nitrite and transform it into nitrate. Hence, there is a correlation between the quantity of fish food consumed and the nitrate production.
Fertilizer
When you start an aquaponics system, you start with almost no bacteria on your media. Hence, there is a whole period when the bacteria are growing. We call this period the cycling process. During the first stages of cycling, ammonia accumulate and then nitrite do. Hence, nitrate are not present in the water before a long time. Some impatient people want to add the plants at day 0. Hence, they add plant fertilizer into the water. The plant fertilizer is actually composed of nitrate. This is not a practice that I recommend.
Impact of nitrate on fish?
Fish are not affected by low nitrate concentration. However, a high nitrate level can generate the following issues:
Fish deformity
High nitrate levels can interfere with fish development. The backbone can form a coma or an “S” shape. If the nitrate level remain high for a long period, fish can have many types of overgrowth.
Abnormal behavior
Affected fish can present strange behavior. Some will swim abnormally fast. Some swim on the side. Others will remain at the surface, trying to breath the air (gaping).
Poor feed conversion
High nitrate levels stress the fish. Hence, they can’t digest the food properly and the feed conversion ratio is affected. As a result, the productivity of the aquaponics system decrease.
Poor growth
High nitrate concentration can affect the fish growth. In long exposition, the affected fish sometimes completely stop growing.
More disease
Nitrate stress consume the fish energy. Hence, fish have less resources to fight against pathogens. As a result, they are more subject to disease.
In short, high nitrate levels can have a dramatic impact on fish health. For your fish welfare please keep a good water quality with an acceptable nitrate level.
What action do nitrate have on plants in aquaponics?
Plants can absorb nitrogen under the form of nitrate and ammonium. However, Nitrate is the main source of nutrients for plants. Hence, they are very important for the plants growth. High levels of nitrate above 150 ppm are often beneficial for the plants. But in aquaponics, we aim to find a good balance between the plants, bacteria and fish needs. Hence, we often aim for a concentration of nitrate from 5 to 25 ppm.
What is an acceptable nitrate level for your fish?
Carps, catfish, goldfish and tilapia can leave in nitrate levels above 120 ppm. But it is better to keep them below 50 ppm, with a target below 30 ppm.
For trouts some sources report acceptance of levels above 500 ppm. However, acceptation to such concentration was for a limited time only. Some scientific studies have shown negative effects below 100 ppm! Hence, keeping your trout in low nitrate concentration such as 30 ppm, is better.
Nitrate can be vicious… There is a range of concentration where fish can be slightly stressed. But without showing visible effects. This zone is very vicious because some fish die. But as they die one by one, they don’t necessary trigger the aquaponics keeper reaction. Hence, it is important to check the water quality regularly.
Why do nitrate increase in your aquaponics system?
As discussed above, the quantity of nitrate produced is related to the quantity of fish food used. This is assuming you have well conceived your aquaponics system with sufficient bacteria population.
Hence, we understand where nitrate are coming from. However, why do they sometimes increase?
Well, the plants are consuming the nitrate. If you have too many plants, they will consume the nitrate as soon as it appears and the level will remain very low. But, if you don’t have enough plants, nitrate consumption is lower than production. Hence, nitrate increase. In theory we would like to have just enough plants to keep the system at the good nitrate level. In practice, we use a little trick to balance the nitrate level. Let’ see…
How to balance nitrate in your aquaponics system?
If the nitrate level is too low:
adding more fish
In a well designed aquaponics system, If the nitrate level is too low, you can simply add more fish. By doing so, they will consume more fish food and produce more nitrate.
Before doing anything, make sure you have the right quantity of bacteria. Your system must be able to support the new fish biomass. There is a ratio to respect between the media volume and the quantity of fish. If you are not sure, get the free aquaponics training, you will find all the critical ratio to respect.
adding fertilizer
Some people like to add fertilizer, I am not fan of this method. The aim of aquaponics is to produce food naturally in an ecosystem. Adding fertilizers to the system doesn’t seem that sustainable to me. But it can make sense for a short period of time. When you start with fish fingerlings, they sometimes don’t produce enough nitrate for the plants. But I prefer to get more fish and offer them as they grow.
If nitrate level is too high
Harvest some fish
As seen above, we decrease the quantity of nitrate by decreasing the quantity of fish food added to the system. hence, harvesting some fish will decrease the fish food needs and therefore the nitrate levels will decrease.
adding more plants
If nitrate level is too high, One obvious option is to increase the number of plants in the system. However, the surface of your flood and drain grow-bed / NFT / DWC is somehow limited. Hence, it may require to extend the system’s surface. If you can afford, then please go ahead, you will have a higher production. If you can’t, I recommend to follow the other approach detailed here below (watering the garden)…
watering the garden…
Another option is to use the aquaponics water to water your classic garden. It will lower the water level in your tanks. Hence, you will be able to top up the tank with new water which will dilute the nitrate and lower the concentration.
Conclusion
You now know the tricks to manage nitrate in aquaponics. Use the table in introduction to define your quick action plan. If anything is unclear or if you have a suggestion, please leave your message in the comment section. I will be very happy to come back to you and continue the discussion there.
If you are new to aquaponics, you will probably be interested to discover my six steps to build and manage an Aquaponics system. Click here to access for free! Thanks and good reading 🙂
