Aquaponic Stocking Rates

“How many fish can I fit in a barrel or IBC?” would have to be one of the most common questions I’ve been asked over the years.  Now, this might sound like an easy question to answer, but it is a bit more complicated if you want to run a stable and productive backyard aquaponic system without losing any fish.  

Now you’ll find some folks will recommend using the fish tank volume as a guide to how many fish you can stock in an aquaponic system. While tank volume most definitely comes into play when designing a system, myself and many other experienced aquaponicists feel that it is the wrong advice to give. The reason I believe it to be wrong is that in many cases these folks will leave out the very important roll biofiltration plays in keeping the water clean enough for the fish and plants to thrive.

This is because the grow beds serve as biofilters which process the ammonia released by the fish. ​​

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The wet surface of the media provides a perfect environment for naturally occurring nitrifying bacteria to set up colonies, and it’s these bacteria that oxidise or process the fish waste. This process is what people refer to as “cycling a system”.

If you’re new to aquaponics you might want to check out our “What is Aquaponics & How Does it Work?” clip as it covers how the bacteria are the backbone of the system in a bit more detail.

The best method I think a newcomer can use to calculate the amount of fish a media based grow bed system can support is by working out the total volume of wet media you will have in the beds.

I need to quickly point out that the following equation is a general rule of thumb used for over a decade now by many aquaponic backyard hobby farmers and not something I have come up with myself. It also has a bit of wriggle room built in so to speak.

The first step to figuring out the “How many Fish?” question is to work out how many grow beds your system will have.

For this example, we'll use 3 grow beds made from recycled IBCs to work out how much biofiltration we will have to process the waste in the water column. 
To do so we must work out the total combined volume of wet media these beds will hold.

The dimensions of the IBC grow beds I’ve cut are 1200mm long X 1000mm wide X 300mm high which is roughly 47.25” long X 39.4” wide X 11.8” high for you folks in the USA.

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Before we work out the volume we need to remove 25mm or 1” from the total height of the grow bed to allow for a lip to stop the media falling out & another 25mm or 1” to allow a dry media zone to prevent the growth of algae. That gives us a total of 50mm or 2” that needs to be removed from the total height of the grow bed.​

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Removing the 50mm now gives us a grow bed that is 120cm long X 100cm wide X 25cm high which gives us 300,000 cm³.

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To convert 300,000 cm³ into litres you divide by 1000, as there’s 1000cm³ in 1 litre. That gives us a total wet media volume of 300L.

Now for you folks in the USA., after making allowance for the lip & dry media the grow bed dimensions are 47.25” long X 39.4” wide X only 9.8” high giving us roughly 18244”³.

Now there are 231”³ in a gallon so if we divide 18244 by 231 we get roughly 79 gal of wet media in each grow bed.

Now we know the wet media volume, we can work out how many fish we can stock.

This rule of thumb equation is based on the many commonly used aquaponic medias such as 20-25mm or ¾-1” suitable rock, volcanic rock (known as scoria here in Australia), expanded shale or expanded clay ball media. The rule of thumb states that for every 25L of wet media in the grow bed you can stock ONE fish you want to grow out to 500g. For you folks in the States that works out to be 6.6 gal of wet media in the grow bed for every fish, you want to grow out to 1lb in weight.

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Now we divide 300L of total wet media by 25L and that tells us we can stock 12 fish we would like to grow out to a harvest size of 500g per bed.
For you folks in the States, we need to divide the 79 gal of total wet media by 6.6 gal and we get the same result of 12 fish that we would like to grow out to a harvest size of 1lb per bed.
Now we multiply 3 beds by 12 and we get 36 fish that I can grow out to 500g or 1 lb in size for this size system.

As I said before, this equation does have some wriggle room built in to allow for unforeseen issues.
One example of this may be if the bulky root mass of larger, older plants restricts the water flow in the beds. The roots of plants will catch the solids which will quickly build up. This build-up of solids stops water and oxygen circulation resulting in anaerobic zones. This, in turn, can lead to a dangerous spike in nitrites and ammonia as well as the release of other toxic compounds. If left go unchecked for extended periods it will most likely kill some if not all of your fish if proper precautions aren't put in place.

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Using this rule of thumb you will have some extra biological surface area so that if an event like this occurs your fish will be safe until you are able to rectify the issue.​ 
You can learn more about my take on Managing Solids in Aquaponics in this article

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I personally think that no matter what size system you have you need some form of solids filtration to prevent the beds from fouling up with solids waste from the fish.

The most efficient DIY filtering device is a radial flow settler, also known as a radial flow filter.​

As I mentioned before, fish tank size has been used by some to gauge how many fish can be stocked in a system. For me, I like to stock no more than one fish I want to grow out to 500g / 1lb per 20L / 5.5 gal of water. You will find commercial systems stock at much higher densities, but they also run commercial grade filtration equipment enabling them to do so.

So as you can see it’s actually quite easy to work out how many fish your aquaponic system can happily house. I do think it’s a good idea to stick to these stocking rates for your first lot of fish just so you can get a handle on running a system.
As I said above, once you gain some experience and pick up some more knowledge, you may be able to slightly increase the fish stocking rate for your system allowing you to grow some extra fish to toss on the BBQ and share with friends.
Cheers all & Happy Growing. 🌱🍓🥦🐟

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