You may need your foil hats on for this one. There is an odd fish stocking density number of 1 fish in 10 litres of water floating about and new comers to backyard aquaculture and aquaponics are becoming baffled about how many fish to get and put in their systems. To complicate this further there has been a few clumsy explanations of why you can stock this number of fish. In this article, I will provide you will a less clumsy explanation why you should not attempt this 1 in 10 stocking density at home.
How many fish you want to grow or how many fish you need is the first thing on your plate. At home, this will depend on how many fish you want to each each week and how many people you are feeding. If you are attempting commercial growth, this will be worked out by how much weight of fish you can sell a year and your cost of production. For this example we will use 50 fish.
The next thing to consider is how big you want to grow your fish. Let’s say you like to eat fish at 500 grams each, this is a general market size for most species. Some are smaller some are bigger, but you get to choose so it is up to you. If you are growing fish to sell, the size will be driven by what you customers want to buy.
Then the tricky part is what weight of fish you will stock per volume of water. This is the stocking density factor expressed in kg/m3 or lbs/f3 (kg per 1000 litres or lbs per US gallon), depending on which side of the planet you are on. Converting backward and forward between these is simple multiplication. To get lbs/f3 = kg/m3 by 0.0624 and to get kg/m3 = lbs/f3 multiplied by 16.02.
It is difficult to give a broad, general advice on density without knowing how your system is put together. The 3 basic criteria below will improve the amount of fish you can grow and feed well and all 3 are related to the oxygen supply in your fish tank:
- Do you have a minimum fish tank water exchange of 1 to 1.5 timers per hour?
- Do you have adequate aeration either by air stones or water return?
- Do you remove fish solid wastes and uneaten food through the use of simple solids filtration?
- If you have the first one only, a good place to start your adventure is 10kg/m3 or 0.63lbs/f3
- If you have the first and second one, you may be able to use 15kg/m3 or 0.94lbs/f3.
- If you have all three you may be able to achieve 25kg/m3 or 1.56lbs/f3.
Now you have chosen your target density working out how much fish tank volume you need is very simple. We will use 25kg/m3 or 1.56lbs/f3 for continuing our example.
If you are working from known tank volume for example 1000 litres. It is important to work with the actual water volume of the tank when it is operating. Avoid the mistake of thinking a 1000 litre IBC is a 1000 litre fish tank. If you cut it down and then have the water below the top (freeboard) of 100mm your IBC may only contain 850 litres of water.
So using an actual tank water volume of 1000 litres you work out the number of fish like this: (1m3) multiplied by Density Factor (25kg/m3) divided by the weight of fish at harvest (0.5kg) will give you the amount of fish to stock with. This example = 50 fish.
If you are working with the number of fish you would like to grow and want to know what tank volume you need use this: Number of fish (100 yes I am going use the magical number) multiplied by the harvest weight of the fish (0.5) divided by the density factor (25kg/m3). In this example the answer is 2m3 or 2000 litres.
There are a range of other environmental conditions that dictate how many fish you can stock your tanks such as bio filtration but I am assuming you have a grow bed or a purpose build bio filter for that. If not, are you mad or does your foil hat need replacing lol? Sticking to the basics of 1 fish in 20 litres of water or less will see you harvesting plenty of fish while the 1 in 10 guys run out of wacky tobaccy bushes to bury their fish under.
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