Stocking Densities in Aquaponics

A regular question that people ask when they are preparing to build an aquaponics system is just how many fish should they put into their tank? This can be a difficult question to answer in a clear cut way. Many of the older seasoned pro’s say “Stock to your level of filtration.” which is a correct response but difficult for the beginner to grasp. How do you know what your level of filtration is if you are building your own system? And to what degree should you engineer your filtration system. How many gravel grow beds do you need? What about swirl filters?

A lot of these questions remain unanswered and for a good reason. Not all fish are the same. Some foul the water a lot faster than others. Some require a heavier oxygen demand and a different water temperature and all these parameters play their part in understanding the requirements of your fish. Speaking with Aquaponics guru Murray Hallam about this issue, he remarked that most people experienced in Aquaponics suggest you should have around 30 fish per 1,000 litre fish tank.

This is a good benchmark figure for all round stability. It also depends how well your system functions and the degree of dissolved oxygen that is created by water flow that is returned to the tank – and whether you have additional aeration supplied through air stones and the size of your filters – the grow beds that contain the plants you are growing.

Figure B.
Murray tends to over engineer his aquaponics system because he knows that people want to grow more fish numbers than their systems are capable of. Its a natural tendency to buy more fish than you need. Now it will come as a shock to many people to learn something that Murray Hallam told me. He has 300 Jade Perch fingerlings in a 1,000 litre tank! They are all doing very well thank you. Now the purists will shrink in horror to learn that fact. But the point is that they are all very small and Murray’s system is robust enough to handle the load of 300 fingerlings – while they are small! It would be madness to expect to grow 300 fish to maturity and expect them all to live in a 1,000 litre tank. Because Murray also has a number of other aquaponics systems, those 300 fish will be redistributed out amongst other systems as they age and grow in size. They will not all be left to grow to maturity in the one tank.

Figure C
Murray likes to point out that fish do not necessarily grow at the same rate. There are some fish inclined to eat more than others and will grow at a faster rate. These fish are the ones that you will need to “introduce to the barbecue plate sooner than later.” So culling your fish and getting into the habit of harvesting them as they grow to plate size is a good habit to get into. Plate size is around 300 grams.

Figure D
So how many fish should you keep and be able to reach maturity in a 1,000 litre tank? As many as your filtration system will allow. What is your water clarity like? Is it pristine and clear? Or murky and muddy and cloudy looking? Do you overfeed with lots of uneaten fish food floating around adding to the buildup of murky water? If you have such a system with only 10 fish and are experiencing such problems – then you have reached your limit of filtration. Time to have a rethink on your fish management skills and improve your design.

Just for the record in our Maximus aquaponics system we had just under 60 Jade Perch in a 1,000 litre system that were harvested recently. All were between 300 and 500 grams in size. Water quality was excellent. So the old maxim of 30 fish per 1,000 litre was easily abled to be doubled. So how many fish should you get? That does depend on your level of filtration and when you intend to eat them.