Aquaponics consist of growing fish and plants in an ecosystem. In this article we will focus on the aquaponics fish. Here is what you will learn:
  • The role of fish in aquaponics
  • Choosing the right fish species of fish for aquaponics
  • The fish tank
  • What quantity of fish
  • Maintenance of your aquaponics fish tank
  • Aquaponics fish care
  • Aquaponics fish food
  • Best time to add fish
  • Where to buy fish
  • How to harvest fish
  • Can we customize the fish tank?
  • Alternatives to fish for aquaponics
 

The role of fish in aquaponics

 
For some aquaponics enthusiasts, growing fish is the main point of aquaponics. Others focus more on the plants. One thing is sure, fish are important for the whole aquaponics ecosystem. As we feed the fish, they reject waste into the fish tank. The water pump pushes the water loaded with fish poop to the aquaponics grow-bed. There, the bacteria are transforming the fish waste (ammonia) into plants nutrients (nitrate).
As you can understand, the aquaponics fish are one of the crucial elements of the aquaponics ecosystem. In other words, they indirectly provide nutrients to the aquaponics garden.
NB: The nutrients provided by the fish in aquaponics are different to fish emulsion. Fish emulsion is produce from fish processing. It is the left over after removing most proteins and fat. Fish emulsion is sometimes used as a plant fertilizer. However, the composition is very different to aquaponics water.
 
 

Choosing the right fish species of fish for aquaponics

 
It can seem like an overwhelming task to select the right species of fish for your aquaponics system. The options are multiple but how to find the best fish for your aquaponics system?

Temperature

First do some research and find what type of fish are adapted to your climate. Indeed, water temperature is one of the most important factors. While a greenhouse could help, the fish you select must be compatible with your local temperatures. If you live in a cold climate, you will choose cold water fish like trouts or perch. If you live in a warm country, you will probably work with tropical fish like tilapia or barramundi.

Growth

Then other factors are to consider. If you already have a good level of experience with aquaponics, you may want to chose fish with a fast growth rate. However, if you are an aquaponics beginner, I recommend to choose a hardy variety of fish. In other words, a robust breed able to tolerate average water condition. That is to say relatively high ammonia and nitrite with low oxygen concentration.

Behavior

The fish behavior is generally linked to his level into the food chain. Hence, omnivorous fish are great for beginners. Those fish are more social and you can sometimes mix the species in a same tank. and more territorial predator fish will be better for experienced aquaponics growers.

Edible

If you plan to eat your fish, you will select edible fish species. Trouts and barramundi are among the best edible fish that you could farm in aquaponics.

Ornamental

If your aim is to keep fish as pet and you want to be able to see them and enjoy their colors, Ornamental fish will be better. The most famous are Koi and goldfish because they accept a wide range of temperature. If you live in a warm country you can keep betta fish and many other varieties generally kept in aquarium.
 
Here below is a table of some of the best fish for aquaponics and their specificities:
10-great-aquaponics-fish
 

The fish tank

Avoid corners if you can 

In aquaponics we keep our fish in a fish tank. This one is generally of a rectangular box shape. Indeed, it is the most simple shape to build. However, from a courantology point of view, it is better to keep the fish in circular tanks. It is also possible to use aquariums as fish tanks but the light coming through the glass can allow algae to grow. We don’t have this problem in classic fish tanks covered with a lid.

size matters…

Stay away from little fish bowls or mini-aquaponics systems. Yes, I agree, those indoor systems look cool. However, they are too difficult to manage due to their small volume. 9 times out of 10 the fish will unfortunately die.

So you may wonder what makes a good fish tank?

You can recycle all sorts of old materials such as containers, bathtub, plastic tanks, little pools… There are no limits to creativity but there are a few rules to respect:

Flood and drain aquaponics

  • Make sure the material is in good structure.

The beginners should probably start with a flood and drain aquaponics system. In this case, the easiest design is to seat the growbed on top of the fish tank. Hence, the fish tank must be sturdy enough to hold the weight of the growbed on top. Another option is to use a support for the growbed.
  • Avoid metals

Metals slowly release ions into the water. At a certain concentration they affect the fish, bacteria and plants. In aquaponics we work in a closed water system. Hence, the concentration of metal ions will only increase with time. For this reason I recommend to stay away from metal tanks in aquaponics.
  • Avoid concrete

Concrete tanks generally increase the water pH. It is more difficult to manage an aquaponics system with a high pH. For this reason, I recommend to not use concrete tanks. If you have already designed your aquaponics system with concrete, do not stress. There are some rubber paint on the market created to water proof water tanks and fish ponds. They will seal your tank and avoid the contact between your fish tank and the water.
  • Make sure it’s food grade

By food grade I mean, you must make sure that no chemical have been in contact with the tank. for instance, you can recycle an Intermediate bulk containers also known as IBC. However, you must check what was the previous purpose of this IBC. If it used to store chemicals such as mineral oils or resigns, do not use it. If it was a vanilla aroma orvegetable oil container, then it’s all good.
  • Make sure the dimensions are in line with your project.

I recommend to have a fish tank approximately twice the size of the grow-bed. Also, I will give you more information on the quantity of fish you can stock in the following paragraphs. So make sure that the fish tank you get is large enough to achieve your aquaponics aim.
Finally, if you want to emphasize the ornamental aspect of your aquaponics system, you can use a fish pond. For me the easthetic of aquaponics is as important as the efficiency of the system. Hence, I always consider the best design to bring harmony in the backyard.
Be aware that you can have multiple fish tanks. Each with different species in one aquaponics system if needed.
 

Equipment around the fish tank

In a simple flood and drain aquaponics system without sump tank, the fish tank will have a water pump. The water pump is pushing the solids to the growbed. Be careful when you select your water pump to have a proper filter in place to avoid the small fish to go through.
I also recommend to add an air pump to the fish tank. It will fulfil the oxygen requirements of your fish.
If you have cold winters and the low temperature is close to your fish limit, it might be a good idea to add a tank heater.
Also, depending on the fish species and fish quantity, you can add a waste filter after your fish tank.
 

What quantity of fish?

Calculating the maximum quantity of fish you can stock is the first thing to do before adding any fish. Each aquaponics system is different. It can be difficult to calculate the exact maximum capacity of your system. However, In this paragraph I will give you the calculation I use to fix a maximum fish quantity limit. But before, let’s define what we mean by fish quantity: It is the number of fish times the average weight of the fish. In other words, this is the total weight of all the fish in your aquaponics system. We call this weight the “fish biomass“.

Fish/Grow-bed ratio

In aquaponics we have one main factor limiting the fish biomass. It is the capacity of the bacteria to transform fish waste into plant nutrients. Indeed, the fish waste (ammonia) is highly toxic for the fish. However, the plant nutrients (nitrate) is many times less toxic for the fish. Also the plants growing on the grow-bed are consuming it. If they don’t consume everything we can use this water as free nutrients for the classic garden.
Hence, we focus on the fish/bacteria ratio.
The quantity of bacteria present in an aquaponics system depends on 2 factors:
  • Media quality
The bacteria are living on the surface of materials. An efficient media will be porous so the bacteria can live on the surface but also into the media.
  • The grow-bed volume
The more grow-bed volume and the more media which means more bacteria.
If you are working with an efficient media, I recommend to not go over a maximum of 1 kg per 50 L of grow-bed. In the above paragraphs, we discussed the fish tank/grow-bed ratio. As a reminder, the fish tank should be twice the size of the grow-bed. Hence, the fish density into the fish tank should be maximum 1 kg/100 L. If you want to stock maximum 5 kg of fish, you will need a fish tank significantly larger than 500 Litres.
Please note that this value is a maximum density, this is not the quantity of fish that you must start with. The fish are constantly growing. So you will start with a lower stocking density, just enough to cover your plants need.
 
So now that we have calculated the maximum quantity of fish, what is the minimum quantity of fish needed?
We know that plants must have access to nitrate to grow. A concentration of 30ppm of nitrate is acceptable for most plants. Even if some prefer it slightly higher.
My recommendation is to not worry about the minimum biomass quantity. Just wait for the nitrate level to reach 30ppm before planting your plants. However, I know that most of you are eager to produce a nice vegetables crop. So here are some values in terms of the minimum aquaponics fish/plant ratio:

Aquaponics fish/plant ratio

How many fish do you need to fulfill your aquaponics plants need? If you are starting aquaponics, it’s a fair question. The system is young without organic matters into the media so my advice is to start with few plants and few fish. But what is a few?
The plants consume nitrogen produced form the fish waste. The quantity of fish waste is related to the quantity of fish into the fish tank and a multitude of factors such as:
  • The water temperature
Each fish species has an optimum. outside of this optimum the fish can survive but their appetite decrease
  • The fish species
Some fish species convert the fish food better than others who create more waste.
  • The fish size
Small fish eat more than larger fish in percentage of their weight
  • The fish food quality
Some fish foods are more digestible than others
In theory, a planted grow-bed request 10 to 25 g of fish food per square meter per day. but in reality, you will probably start aquaponics with a low density of seedlings plants. Those consume very low quantity of nitrates. Hence, a quantity of 7 g of fish food per day can be enough to start an aquaponics system with a grow-bed of 1 square meter. Also, we generally start aquaponics with small fish (called fingerlings). If the water temperature is in the fish optimum, they will consume around 3% of their body weight per day. Hence, you can start aquaponics with only 230 g of fish per grow-bed square meter. Again, this is a minimum quantity and you are welcome to stock more.
If you have a flood and drain aquaponics system, the recommended media height is 30 cm. Hence, one square meter of grow-bed contains approximately 300 Litres of media. the minimum fish biomass is therefore: 38 g of fish per 50 L of media (300 L / 50 L = 6 230 g / 6= 38 g).
Disclaimer: The above limits are indicative only. The fish food intake of a fish can go from 0 to 5% depending on the above factors. Hence, the values above are only averages and can considerably vary from one case to the other.
 

Maintenance

Fish tank

In a classic aquaponics system, the fish tank maintenance is relatively limited. Unless you use an aquarium as fish tank, we don’t mind to have a biofilm on the wales. It is actually a positive diversity for the whole ecosystem. Hence, we will not spend time cleaning the wales as in a classic aquarium. The main maintenance of the fish tank is to avoid any buildup of organic matters or “mud” on the bottom of the tank. If you have enough fish and the water pump is strong enough, the water is well sized, you shouldn’t have to do anything. However, if you see any mud on the bottom of the tank, it is good practice to pump it out or to siphon it away.

water pump

The water pump sometimes need a little clean. Some water pumps are better than others and may not require much work. But it’s important to check that the water flow is constent and not decreasing.
 

Fish Care

Your aquaponics fish require attention. However, as they live in an ecosystem, they live in clean water and don’t have many chances to fall sick. Nevertheless, it’s good to check on your fish every day when you feed them. Hence, I don’t recommend automatic feeders. Feeding the fish give you the opportunity to check their behavior. A sick fish will generally not eat. So if you care for your fish welfare, make sure to observe them when you feed them.
Winter can be a sensitive time for some fish. Make sure the temperature doesn’t drop to low and remain in their thermal preferences.

Water parameters

If you see a fish in poor conditions or you spot a dead fish, immediately do a water test. It may just be a one off but if you see your ammonia/nitrite or even nitrate above the normal range, renew some water.
What are good ammonia, nitrite and nitrate values for your fish?
Ideally we would like to see those 3 values at zero for the fish. However, as they live in an ecosystem and the plants need nitrate, we generally fix the limits below:
Ammonia: maximum 0.5ppm (part per million = mg/litre)
Nitrite: maximum 0.5ppm
Nitrate: maximum 50ppm
The pH is more relative to each fish species but in aquaponics we maintain it between 6.4 and 7.6

Fish Disease

If your water parameters are OK and you respect the fish density, your chances to find your fish sick are low. However, it is interesting to know the 3 common types of fish disease:
-Virus
Virus are common in high density aquaculture. However, the chances to get a virus on your aquaponics fish are fairly low. They generally prefer low temperatures.
-Bacteria
Bacteria infections are very common in aquaculture. Some fish farms chose to vaccinate the fish one by one. Again, in aquaponics if you respect low fish density, you will probably not have this problem. Bacteria disease prefer warm water.
-Parasites
There are different types of parasites. Among them we have the nematodes, crustaceans and fungus. Nematode parasites are often found in the muscles or internal organs of the fish. Nematodes are very common in wild fish. However, they have never been found in closed aquaculture systems or aquaponics.
Crustaceans and fungus can live on the surface of the fish, fixed on the skin, fins or gills. They are more common and will take advantage of the fish weakness to grow. Hence, respect low fish density and avoid fish stress to avoid those parasites.
If you spot some parasites on your fish, salt bath are a good way to solve the issue. You will find more information on this article about white spot disease.
Do your fish need light?
Finally, like humans, fish need sun light to transform cholesterol into Vitamin D. Therefore, if you cover your fish tank to avoid algae, make sure to leave some sunny area to allow the fish to sit in the sun.
 
Fish food
You may wonder, “What do aquaponics fish eat?”. In aquaponics we have a whole ecosystem. The fish waste feed the bacteria, then the bacteria waste feed the plants but who is feeding the fish?
Unfortunately, we haven’t found a sustainable and convenient way to close the loop. Many growers are trying to feed the fish with crop that they grow in or aside the aquaponics system. I personally grow duckweed and worms that I enjoy to feed my fish with. Others are growing black soldier fly larvae from their plant waste and feed the fish with. Even if those homemade fish food systems exist, most of us are still using aquaculture fish feed. Food alternatives are interesting but they are not as convenient as using dry pellets. The aquaculture fish food is generally made from fish meal and vegetables. It is very efficient and dry. Hence, it can be stored for months and the results are impressive. The feed conversion ratio for rainbow trout is 0.9. In other words, we need only 900 g of dry fish food to produce 1 kg of trout. The largest fish food suppliers also offer organic certified fish feed.

When should you add the fish?

Here is a crucial point. The short answer is one month. But this response is just an average as each aquaponics system is different. Too many beginners make the mistake of adding fish to the fish tank on day one. Others are asking if they should add the fish or the plants first. if you ever had an aquarium, you know that we must wait before adding the fish. Now what are we waiting for? We are waiting for the bacteria to colonize the media. You remember that we are working with an ecosystem composed of fish, bacteria and plants. The bacteria are very important. They transform the toxic fish waste into plant nutrients. Without bacteria, the fish waste will build up into the water and the fish will eventually die. When we start an aquaponics system, we don’t have the good population of bacteria on our grow-bed media. Bacteria are everywhere. However, in aquaponics we need specific bacteria at a high density. Hence, we offer good conditions for those bacteria to grow but we must offer them enough time to grow. We call this process “cycling a system”. To cycle an aquaponics system I recommend the “fishless cycle”. In other words, we lave the aquaponics system empty of fish while the bacteria are growing. The aim is to get a bacteria population able to transform the future fish waste into plant nutrients. The trick here is to add a little handful of fish food into the fish tank the first day. Then, to test the water frequently (every 3 days is a reasonable frequency). By testing the water, we can measure the concentration of the different forms of nitrogen. At day 0, the fish food is slightly breaking down into the water. We detect a very low concentration of ammonia. no nitrite or nitrate. in the middle of the cycling process, we will identify a peak of ammonia and nitrite. They are toxic elements for the fish. This peak is the reason why we don’t add the fish from the beginning as some would not survive those conditions. At the end of the cycling process, the water quality becomes good for the fish. Ammonia and nitrite are at 0 and the plant nutrient (nitrate) start to raise. This is the good time to add the fish and the plants. This cycle generally takes a month. However, this timeline is just an average. it really depends on heaps of parameters such as pH, water temperature, media quality… Hence I recommend to test the water and see the 2 peaks by yourself. Then, once the toxic elements are back to 0, this is the good time to add the fish.

Where to buy fish?

First you must do your research and check that the fish species you want is legal in your country/area.
When we stock a fish tank we often add small fish. Indeed, little fish have plenty of potential to grow. We call them fish “fingerlings” because they are the size of a finger.

Hatchery

The best fish fingerlings suppliers are the aquaculture hatcheries. It’s not always easy to find them but you can get in contact with your nearest fish farm. Ask them if they offer fingerlings or if they know an hatchery doing so.
Another option is to get fingerlings through internet. Sometimes the hatcheries offer to pack and post the fish directly to your address.

Tackle shop

You may try your luck in a tackle shop and see what they have as live bait. They sometimes offer live fish or feeder fish.

Aquarium

Aquariums are also an option but unfortunately, it’s often expensive
to stock a fish tank from them.

Wet markets

If you live in Asia, you may find the possibility to buy live fish in the wet market. You will also be able to make connections with farmers or fisherman. They could get you the targeted species at the right size.

Local dam

Finally if none of the above options are available, you can fish your own little fish from your local dam or rivers. It may not be the exact species you had in mind but at least it will be adapted to your local climat.
 

Can you have plants in your fish tank?

Many of us are inspired by ornamental fish ponds to add plants directly into the fish tank. And to the question is it possible? The response is “yes!”.
Aquatic plants will be able to grow in your fish tank at two conditions:
1. Make sure your fish or crustaceans will not eat them
2. Make sure they have access to enough sunlight.
However, you must be aware that even if it is possible, from a maintenance perspective it will add more work load. Indeed, you will have to trim the plants. Also, the water pump maintenance will be more complicated as the leaves will easily clog it. Finally, seeing and catching your fish will be more challenging. For all those reasons, we don’t often see aquatic plants in the aquaponics fish tank.
The other option is to grow vegetables on rafts directly on top of the fish tank. This option is rarely a good idea for 2 reasons:
1. Some fish like to eat the plant roots.
2. Solid fish waste often floculate around the roots and decrease the aeration efficiency.
Hence, it is better to keep the DWC in separate tanks and use a filtered water.
 

Alternatives to fish for aquaponics

Yes, we can use other aquatic animals such as crustaceans or mollusks. However, I would recommend to be careful if using turtles as they are reptiles. They sometimes carry pathogen bacteria that you don’t want to find on your crop. Same comment for warm blood animals such as ducks. You can still take the decision to include them in your aquaponics loop. But I would recommend to wash the crop before consumption.

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 🙂