Having high fish stocking density means having high risk in aquaculture and aquaponics. Even commercial producers avoid high fish stocking density where the risk outweighs the return. Certainly home producers are best to avoid it at all costs as it simply is not worth it.

Online, many talk about a high fish stocking density number of 1 fish in 10 liters of water. This number of fish is achievable in quarantine systems with fingerlings and tank space to grade them out into. However, you will not keep them alive at high density for long unless you have serious filtration.

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 100 fish in 1000 liters at home.

So how many fish should you have?

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. A fish weight of 500 grams is a general market size for many species as they are considered “plate size”. 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 as kg/m3 or lbs/ft3 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/ft3 multiplied by 16.02.

“For those math challenged, Google is your friend. If want to use lbs per gallon, there are 7.48051948 US gallons or 6.22883288 Imperial gallons per cubic feet of water. You can see as a general rule I stick with metric.”

Avoid high fish stocking density!

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:

1. Do you have a minimum fish tank water exchange of 1 to 1.5 timers per hour?
2. Do you have adequate aeration either by air stones or water return?
3. Do you remove fish solid wastes and uneaten food through the use of simple solids filtration?

1. If you have the first one only, good water exchange then 10kg/m3 or 0.63lbs/f3 will be ideal.
2. If you have great aeration and water exchange then 15kg/m3 or 0.94lbs/ft3 will work well.
3. If you have all three you may be able to achieve 25kg/m3 or 1.56lbs/ft3.

“You will be surprised how much more productive your system will be with the addition of solids filtration. Even basic ones.”

Above 25kg/m3 is certainly achievable provided you tick all the boxes and have improved solids filtration. As a beginner, you may want to leave the higher densities until you have grown out some fish and you understand your system and its limitations.

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/ft3 for continuing our example.

How to work it all out

If you are working from known tank volume for example 1000 liters. It is important to work with the actual water volume of the tank when it is operating. Avoid the mistake of thinking a 1000 liter IBC is a 1000 liter fish tank. If you cut it down and then have the water below the top (freeboard) of 100mm your IBC may only contain 850 liters of water.

Working with tank volume

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.

Working with a number of fish

Number of fish (100) multiplied by the harvest weight of the fish (0.5) divided by the density factor (25kg/m3). You will need at least 2m3 or 2000 liters. More is better.

There are a range of other environmental conditions, fish size and condition that dictate how many fish you can stock your tanks. Such as bio filtration but I am assuming you have adequate grow bed space or a purpose build bio filter sized correctly for that.

Sticking to the basics of 1 fish in 20 liters of water or less will see you harvesting plenty of fish. Always add more tanks and keep your fish in the best possible environment and they will always produce a wonderful product/meal.