Aquaponic Stocking Rates

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“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.  

It's ALL about the Bacteria

While some folks will recommend using the fish tank volume as a guide on how many fish you can stock, myself and many other experienced aquaponicists feel that it is the wrong advice to give.

In fact, it is the size of the biofilters/grow beds that dictate how many fish can be raised in a healthy backyard aquaponics system. 

This is because these biofilters/grow beds are home to the bacteria that will process the ammonia waste generated by the fish.

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In most backyard aquaponics systems, it is the wet surface of the grow bed media that provides the perfect environment for naturally occurring bacteria to set up their colonies. It’s these bacteria that oxidise/process the toxic ammonia and nitrite into plant available, and fish friendly, nitrate. 

Setting up this mini nitrogen cycle within our aquaponics system is what often called, "Cycling a System".

If you’re new to aquaponics, you might want to check out these two informative videos that look into the roll bacteria play in aquaponics & how to "Cycle" your own system.

Crunching the Numbers

The best method 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 well over a decade now. It is used by many backyard hobby farmers and also has a bit of "wriggle room" built in, so to speak.

So, 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 ammonia waste generated by the fish. 
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 of the grow bed we need to remove 25mm or 1” from the total height of the grow bed to allow for the lip that stops the media falling out & another 25mm or 1” to allow a dry media zone, which prevents the growth of algae on wet media at the surface of the bed.

That gives us a total of 50mm or 2” that needs to be removed from the total height of the grow bed.​

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.

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.

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We can now multiply the number of beds (3) buy the volume (300L/79gal) which gives us a total volume of 900L or 237gal.

Calculating Stocking Density

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

This equation is based on the many commonly used aquaponic medias, such as 20-25mm or ¾-1” suitable rock, volcanic rock (also known as scoria), 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 that you want to grow out to 500g.

For you folks in the States that works out to be 6.6gal of wet media in the grow bed for every ONE fish, you want to grow out to 1lb in weight.

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So for the system in the example, we divide 900L of total wet media by 25L, and that tells us we can stock 36 fish that will grow out to a harvest size of 500g. 

 

For you folks in the States, we need to divide the 237 gal of total wet media by 6.6 gal and we get the same result of 36 fish that can grow out to a harvest size of 1lb.

"Wriggle Room"

As I said before, this equation does have some wriggle room built into it, 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 these plants can collect solids, which can stop water and oxygen circulation, resulting in anaerobic zones. This, in turn, can lead to a possible dangerous spikes in pH, nitrites, and ammonia, as well as the release of other toxic compounds.

If left go unchecked for extended periods it could kill some, if not all of your fish. There are some precautions that can be put in place to reduce these risks.

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Using the above rule of thumb to calculate stocking rates, you can provide the system some extra biological surface area, so that if an event like this does occur, your fish will be safe until you are able to rectify the issue.​ 

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Regardless of system size, having some form of solids filtration is best to prevent rock/clay based 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.​

In this article you can learn more about Managing Solids in Aquaponics

As mentioned previously, 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 (max. growth 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.

As you can see, it’s actually quite easy to work out how many fish your aquaponic system can happily house. It is 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, and as your knowledge and experience grows, you can always add more fish later.

Cheers all & Happy Growing. 🌱🍓🥦🐟
Rob

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