Selecting the best breed of fish to grow out in your aquaponics system is an easy task for most people. There are a few things that must be considered when selecting your fish. Some do not fare well in an enclosed environment, such as your tank, and you must also consider the climate and dietary requirements.
Consider the growth rate of your intended species as if it takes years for the fish to grow out to a size considered suitable for the table. There is a point where it becomes uneconomical to feed the fish for that period of time.
Some people catch their fish in the wild when they are small and stock their aquaponics system yearly from natural stock. This practice is a bit questionable in some ways, particularly in instances where local legislation prohibits the removal of small fish from the wild or when a disease is introduced into your system from less than healthy fish.
For the most part, your average aquaponic gardener will likely purchase their stock from a fish hatchery, and this is the path I prefer to take as fingerlings purchased from a hatchery tend to be disease-free. The hatchery may also be able to provide you some advice as to what species are best for aquaponics and supply you fish that are likely to be suitable for your climate.
In most places worldwide, tilapia is the most common fish picked for both home and commercial aquaponic projects. There are nearly a hundred different tilapia species, and in the wild, they inhabit freshwater habitats from shallow streams and ponds to rivers, lakes, and estuaries.
Because of their large size, rapid growth, and suitability for the table, tilapia are ideal for aquaculture. They are one of the three most commonly utilized fish in commercial fish farms. Tilapia are omnivorous and farewell on fish pellets and soft aquatic vegetation.
Tilapia may not be suitable for your climate or not permitted by local legislation. For example, tilapia is banned in all, but native fish are the most popular aquaponics in one state in Australia. Common choices include sleepy cod, jade perch, silver perch, barramundi, and Murray cod.
While brown trout and rainbow are not natives, they are selected in the colder regions. Consult your local hatchery for the species that is most suitable for you. Most hatcheries will endeavor to be as helpful as possible if you are not wasting their time and intend to buy something. Find a good hatchery and stick with them, and you will find they will appreciate your repeat business.
Feeding Your Fish
Different species prefer to eat different things; however, there are commercially available fish pellets to suit the dietary requirements of most fish. Some pellet manufacturers even go so far as to provide pellets tailored to the age and size of the fish.
Most fingerlings require more protein than older fish, and starter pellets are generally smaller at around 1mm and are more suited to their requirements. As the fish grow larger, their needs change, and they prefer a larger pellet.
Pellets are usually available in several sizes, from 1mm to 4mm, and some float on the surface while others sink quite quickly to suit the feeding habits of the species you are growing out of.
Commercial fish pellets will provide optimum nutrition for your fish. However, there is nothing wrong with using more natural feeds though you may find that your fish grow more slowly as a result. Worms, bugs, and soldier fly larvae are the most commonly used natural feeds.
I like to go through my garden, pick the pest insects of the leaves, and put them in an ice cream container. When I can’t find any more pests, I feed these collected bugs to the fish and leave them for 20 minutes to feast. I then finish with native fish pellets, ideal for the silver perch I am currently growing out.
Overfeeding fish is a common mistake, one I have made and still occasionally make myself. It is surprising how little fish really eat, and if there is uneaten food in the tank, it will break down into ammonia and contribute to poor water quality.
This can be useful if your fish are small and don’t poop enough to provide enough ammonia to be converted into nitrate to feed your plants, however, if your fish is big, your stocking rate is high, or you don’t have enough grow bed area this can make a bad situation worse.
Watching the fish as they eat can provide a clue as to how much to feed them. When they are hungry, and you drop food into the tank, you will see that they run for the food with vigor, but after a few seconds of feeding, this vigor subsides, and they eat at a more leisurely pace. Aim to provide them the amount of food they would eat vigorously and no more.
How Many Fish?
How many fish you can grow is referred to as your stocking rate. Determining your stocking rate can seem difficult because many factors influence how many fish your system can hold.
If your fish are fingerlings, you can have many more of them in your system than you could if they were grown out and ready for the table. The size of your grow bed is important, too, as this determines the level of biofiltration available to your system.
A recommended ratio for your system is 1:1. That is for every 10 liters (approx 2.5 us gallons) of fish tank capacity. It would help if you had 10 liters of grow bed capacity. I would suggest no less than 10 liters of fish tank capacity per fish. So in this example, if we have 1000 liters of fish tank capacity and 1000 liters of grow bed capacity, our maximum stocking rate is 100 fish. Clear as mud?
If you had a fish tank capacity of 1000 liters, but your grow bed capacity was 500 liters, I would not suggest more than 50 fish for this system as the biofiltration is not likely to be adequate for fish to get bigger. With 20 liters of fish tank capacity per fish, you will probably have happier fish too.
The above rule of thumb works reasonably well until your fish start to grow beyond the 500-gram mark. Some fish species can grow well beyond 5 or 10 kg, and the above formula completely falls apart well before that point. A better measurement of your stocking rate is kilograms of fish per liter of fish tank capacity.
Confusing? Not really. As your fish get larger, they will create more fish poop, and they will require an increasing amount of water per fish as a result of this. A fish that weighs 5kg will create far more fish poop than 20 fingerlings.
I have seen many different recommendations for stocking rates from several different sources. To provide some guidelines, 1kg per 30 liters of water is a very safe stocking rate. That is, if your fish all weigh 2kg each and you have ten of them, you should have a fish tank of at least 600 liters.
A stocking rate of 1kg/20 liters is ok, but you must monitor your water quality regularly and avoid overfeeding your fish. If the water quality falls outside of acceptable guidelines, immediately change 1/3 of your tank capacity for fresh water and be prepared to change another 1/3 24 hours later if the situation does not improve.
Needless to say, you should start looking for the cause of the problem. I have grown silver perch successfully at stocking rates as horrifying as 1kg/10 liters with a 2% mortality rate. However, this is running very much on the ragged edge and has absolutely no margin for error.
I consider the secret to this incredible stocking rate to be duckweed, which silver perch absolutely loves to eat. When added to your fish tank by the shovelful, it has the additional benefit of removing some ammonia from the water before the fish get around to eating it.
An efficient solution when your fish have grown, and your stocking rate is beginning to push the boundaries is to start harvesting the bigger fish, which will create more room for the smaller ones. My favorite, silver perch, is best for the table when they are between 1 and 1.5kg.
As you will discover, not all of your fish will grow at the same rate, and there will be a number of them that will be ready to eat long before others. Some selective culling will soon put your stocking rate to rights.
Aquaponics is a balancing act at all times, and testing your water regularly will alert you to any problems as they develop. Avoid the temptation to overstock your tank, particularly if it is your first effort.
Aeration is critical to the survival of your fish. Without an aerator, your fish will breathe the air trapped in the water of your tank in a concise period of time. In fact, with a high stocking rate and adult fish, if your aerator is switched off for more than an hour, you can expect the fish to start dying after that time.
Aerating the water in your fish tank does not need to be a complex affair, as with an aquarium, all you need is an air pump, a suitable length of air hose, and air stone, and your fish will be able to breathe easy. As a rough rule of thumb, your air pump should be capable of pumping twice the volume of air as your fish tank contains water.
For example, if you have a 500-liter fish tank, a 1000 liter per hour air pump should be sufficient. If you want to go bigger, that’s no problem. You cant over aerate your fish tank water as it can only absorb a certain amount of air.
Power failures that result in the air pump not working for extended periods will also cause you grief, and it is recommended that you consider a backup system in case of a power failure.
My aerator is plugged into an uninterruptible power supply designed to keep computers going in the event of a power failure, and it can keep my air pump going for several days.
Another option to consider is a 12-volt pump that can be connected onto a car battery with a short pipe from the pump to above the water level in your tank with a u-bend to direct the flow back onto the surface of the water.
As the flow from the pump hits the surface of the water, it makes a splash that creates bubbles that, in turn, will aerate your tank for you.
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