Fish to plant ratio
Here you, came the time when you were in front of your first ever aquaponics system and you were wondering how many fish can you "stock" in there... For many it is a challenge, for a few it is a gamble and the rest, they follow a certain ratio.
Define your growing space
I have been asked this more than one time and it is not that I pull numbers out of a hat instead, I am following established rules.
The following is working for backyard growers using media based growing systems, either in an IBC or barrel or whatever suits your need. Balance is what we are aiming in order to have a healthy and thriving ecosystem - overload doesn't work.
Tank volume as your base is not a good idea. Aquaponics is about bio filtration and so it depends on the size of your bio filter aka the grow bed. Bacteria will process the waste from the fish and turn it into plant available nutrients. Remember, in a previous post, I talked about the Nitrogen Cycle where I explain the role and nature of the bacteria in our grow beds.
In a media bed, only the submerged portion of it allows for the bacteria colony to thrive so it is this part that is of interest in order to size your stocking density.
For the advanced aquaponists here, you may have heard about Biological Surface Area aka BSA. This is in relation to the media in the grow bed. The whole media surface, in our case clay pebbles, have a very porous surface and holding water in tiny pockets; this is paradise for our nitrosomonas and other nitrobacter.
Onto the calculations
I will use metrics as I am French and don't find any logic in the imperial system but for you, folks out there, I will give you a break and make a rough translation. Note here that my numbers once translated into this weird measuring unit may differ from what you'd get if you spoke the Imperial lingo... Anyways, without any overdue, take a pen, a paper and a measuring tape. Or if you use an IBC type system, you can use my measurements as they come from that model of aquaponics setup.
For the Imperialists out there, you have 47'' in length, 40'' width and 10.24'' high. This gives you a total of 19,251.2''3 (cubic inches). You most likely know that you have 231''3 in 1 gallon, therefore, 19,251.2/231=83.33 gallon.
This is your total submerged media volume.
The rule of thumb
Now that we know our total submerged media, we are able to find out how many fish we can put in our fish tank. Assuming we are using appropriate aquaponics solid media, of course.
So it goes as such:
In Imperial, 83.33 / 6.6 = 12.6 fish ( round it down to be conservative)... So more food for the Metric people! :)
As far as stocking fish in a tank like an IBC, yet another rule of thumb to follow is for every fish you want to grow out to harvest size, you need 25lt of water. An IBC holds approximately 1000lt and so you could stock up to 40 fish in that volume of water.
It is recommended to add fish progressively to your system after it has cycled. The bacteria colony must be looked after and developed enough to sustain the fish waste load you will give it to chew on.
So very important to log your water parameters often at the early stage and on a regular basis once fish are in the system. An ammonia or nitrites spike is lethal for the fish and can happen really fast.
I hope this article will help you start on a good foot and enjoy the reward of a thriving and healthy aquaponics system.
I am a French expat, married, dad of one, nature lover and firm believer of sustainable living. My background is leisure and tourism industry. I fell into aquaponics and now I am sharing with you my adventure.