If you are looking for a short answer on the Aquaponics stocking densities to use, then go for 1 fish per 10 liters of water. Now for the longer answer. It is more accurate to talk about stocking densities in terms of kilograms of fish per cubic meter or liters of waters. For most Aquaponics set-ups, you should go for 30 to 40 kg of fish per 1000 liters. Use this rule of thumb as a maximum number, and opt for a lower number if you are new to Aquaponics. Keep in mind that apart from the stocking density, the amount of food you put into the tanks will also dictate how well the system runs - if you feed too much, there will be an accumulation of waste that the bacteria may not be able to handle.

Perhaps the most common fish used in Aquaponics farming is tilapia. So let's work out stocking densities in terms of tilapia. When you buy 100 tilapia fingerlings, these may weigh a kilo combined. If we have a 1000 liter fish tank, then our current stocking density is 1kg/1000L. Tilapia grow quickly and soon enough they'll weigh a combined total of say 40kg, giving us a stocking density of 40kg/1000L.

By using the rule of thumb of one fish per ten liters of water, then you'll have to harvest the fish as soon as they're big enough, or else you'll end up with a stocking density which is straining the system. Individual fish grow at different rates, and you should start taking out the large ones as soon as they're big enough to eat. Do not wait until all of them are big enough. This is actually a good thing, as you will have to harvest at a rate at which you can eat/sell them and not have to freeze a large quantity of fish. You can actually harvest a single lot of fish over several months, as the rates of growth of individual fish are so different. This applies even to fish species with particularly fast growing rates, such as trout.