The most important questions people ask about aquaponics is, “How many fish should be put in an aquaponic system?”
It may seem to be a fairly easy question to answer, but it’s a little bit more complicated than that if you want to run a stable aquaponics system and not lose any fish along the way.
Many experts recommend using tank volume as your metric as per how many fish you can put in your tank.
Fish to Grow Bed Ratio
Many experts recommend using tank volume as your metric as per how many fish you can put in your tank. But that’s not a very good idea because many people miss out on the all-important biofiltration aspect when only considering the ratio of fish to tank volume.
The best method you can use when calculating how many fish you can run is to base it on the total amount of wet media you have in the grow beds.
The reason is that this wet media is effectively a biofilter that will process the ammonia the fish have released. The wet surface of the media in the beds is a perfect environment for bacteria to set up colonies.
These bacterial colonies will process the waste from the fish, detoxify the water, and make the nutrients available for the plants.
Biological Surface Area
When you delve deeper into aquaponics, you’ll hear this surface of the media called the biological surface area or BSA basically because the surface of the media provides a home for the bacteria to set up shop so they can oxidize the fish waste.
What Do the Bacteria Do?
There are two types of bacteria:
Nitrosomonas convert ammonia, which is toxic to fish, into nitrite.
The nitrite is also toxic to the fish, but that’s fine because of the second group of bacteria, Nitrobacter, which converts nitrite into plant-available nitrate. This nitrite is non-toxic to fish in the level used in sensibly stocked backyard aquaponics systems.
Both these forms of bacteria are commonly found in the atmosphere. As long as you set up the right conditions by adding a little bit of ammonia to the system and a nice damp media for the bacteria to build their colonies, you will have to do very little for your aquaponics systems. This is also known as Cycling a System.
The first thing you got to figure out is how much space you have available and how much space you’ll need for hydroponic or the grow bedside of the aquaponic system. For example, we will talk about the IBC based system.
IBC Based System
For this system, you should have:
- Enough room for three grow beds with enough space to place a tank underneath one of the beds
- An IBC to use as a fish tank
- A bit of space left over for the later addition of filters
Now that you’ve got three IBC grow beds to work with, you can determine how much wet media you will have in total volume.
What is Wet Media?
Wet media is any media underwater during a flood and drain cycle of a grow bed or in a constant flood grow bed. The bacteria need wet media to be able to colonize. So first, you need to work out how much volume you are working with in each bed. Calculating the volume is very easy.
First, you need to work out the volume of one bed and then multiply it by three. Then, you need to multiply the length of the grow bed by the width of the grow bed by the height of the grow bed.
Before you work out the volume of wet media, however, you need to remove some of the height from the grow bed because you need to allow for two and a half centimeters or an inch worth of width to stop the media from falling out of the grow bed.
It is also important to allow another two and a half centimeters or inch of dry media on top of the wet media because if the wet media is exposed to UV light, it will grow algae which can cause problems. Just one issue is the algae robbing nutrients from the system.
To prevent this, you need to remove five centimeters or two inches in total from the grow bed’s height.
Number of Fish
Now that you know the wet media volume in a single bed, you can work out how many fish your bed can sustain.
The rule of thumb states that every 25 liters of wet media we have in the grow
bed, i.e., 6.6 gallons is enough biofiltration to handle the waste from one fish, that is 500 grams or just over one pound in size.
For example, if you have 300 liters of wet media in your grow bed, you will need to divide it by 25. This will come to a total of 12 fish.
This means with this three-bed system (IBC based system), as in the example, we can stock 36 fish that we want to grow to 500 grams or around about a pound in size.
When it comes to tanks, the rule of thumb which one should follow is knocking one fish you want to grow out to 500 grams or a pound in 20 liters of water or 5.5 gallons. That means, in an IBC system, no more than 50 fish should be in there at the same time.
It’s easier to start with a tank of 36 fish. If you want to add extra grow beds later on, you can stock more than 36 in the tank, but it’s recommended to keep the amount to 36 fish for the first crack.
Suppose you have a plant with a large root mass growing in a grow bed that is slowing down the flow of water and eventually starts to collect solids.
In that case, you could end up with an anaerobic zone down where those solids are slowly decomposing and could potentially lead to a dangerous spike in ammonia and nitrite, which could end up costing you some if not all of your fish.
What’s The Solution?
One way you can remove the problem of solids making to the grow beds in the first place is to set up a solids filter between the fish tank and the grow bed itself.
Things to Keep in Mind While Buying Fish for Your System
If you’re growing in a greenhouse or inside or in a northern climate or a tropical climate, this will have a significant effect on your water temperature, which is ultimately going to have a substantial impact on the species of fish that you raise.
So, if you’re in a greenhouse environment, chances are you’re probably going to want to raise a warm water fish.
If you’re in the aquaponics gig for a business, you definitely want to make money selling the fish. You don’t want to have fish as simply fertilizer makers and just consuming your resources and not giving you any profit. So you want to raise a fish that you can sell.