A very common question is: ”How many fish should I put in my aquaponics setup?” and this is what
we will see in this video. When you design an aquaponics setup you must keep some numbers and
ratio in mind. There is a clear limit into the quantity of fish that your aquaponics setup can hold. The
more fish biomass you will have into your aquaponics setup and the more quantity of fish food you
will have to feed them. The more fish food you put into your aquaponics system and the more
ammonia will be released. We therefore easily understand that the quantity of fish able to leave into
the aquaponics ecosystem will be directly dependant on the quantity of bacteria able to transform
the ammonia generated into nitrite and nitrate. The quantity of bacteria is directly dependant on the
growbed volume and the quality of the media selected. Indeed the bacteria population will be
limited by the surface they have to develop on. In this video we will see that the largest the growbed
volume is and the more fish you will be able to stock into your aquaponics setup. If you have enough
bacteria, they will transform all nitrogen present into the aquaponics setup into nitrate. The plants
will be able to consume the nitrate and if you don’t have enough plants into your aquaponics setup
the nitrate will slowly build-up. The nitrate are not as toxic for the fish as the ammonia and nitrite so
you will have time to see the concentration increasing but it is a good thing to keep it below 40ppm.
If you see the concentration over this number you can simply use some water from your aquaponics
system to water your classic garden and top-up your system with some new water which will dilute
the nitrate concentration. So how to calculate the maximum quantity of fish to put into your
aquaponics setup? Well there is not straight and easy calculation as the bacteria have different
yields depending on the water pH, the temperature of the water, the quantity of oxygen available…
but we like to give a rough maximum ratio of 1kg of fish per 50 litres of growbed volume. This is a
maximum limit for the most efficient media so I recommend everyone to always stay way below so
understand this is not a target but a maximum limit to stay under.
Is there a minimum quantity?
The other question is: “what is the minimum quantity of fish required for my aquaponics setup?”.
Indeed the plants food “Nitrate” quantity generated by the aquaponics system will be directly
dependant on the quantity of fish biomass into the fish tank. When you begin your aquaponics setup
you generally add fingerlings fish after cycling the system. The total biomass of those fish (average
weight X number of fish) is generally fairly low. In the same time the plants and vegetables
population is also young (especially if you grow your plants from seeds) so in this case the system is
somehow balanced. If you add a significative quantity of plants from the gardening shop at the
beginning, you may not generate enough nitrate to feed them. In this case we have 2 main options.
The first and most common one is to add more fingerlings and to sell them later on when the total
fish biomass get closer to the limits of the system. The other option is to add some fertilizer directly
into your system (some people add Seasol to their Aquaponics water).