What is a balanced aquaculture or aquaponic system and how many fish can you grow at home in a 1000 liter tank comes up often. Most of the answers I have seen are based around commercial capacities, which are rarely achieved at home.
First of all we need to understand, while these aquatic systems behave in an eco system type, they are completely artificial. Meaning what you do to them plays the biggest role in the entire performance of the system. If you leave them to their own doings, the eco system will quickly die.
The first limiting factor in any fish system is going to be oxygen and nitrogen, If you are growing fish we will assume you have the oxygen and nitrogen under control. In an aquaponic system nutrient accumulation or depletion is the primary limiting factor.
Balanced Aquaculture and Aquaponic System
Use the table below to check you are on track towards balancing your system:
Breaking down the table
All of the numbers in this table refer to a 1000 liter fish tank. So 20kg is 20kg/m3 or 20kg per 1000 liters. The green area is where I recommend you start out, the orange if you have some experience growing fish out and the red… well I would not be too quick to try that out at home.
Number of fish
This is quite simply what we would call a plate size fish – 500 grams. The size of the fish is really irrelevant but it does help you decide on how many fish you put in your 1000 liter fish tank and what follows from there.
Feed per day
This is in grams and based on 1.2% of the total body weight (biomass) of the fish in the system at full load.
This is based on a 50% protein feed. Yes I know many of you will not be feeding such a high protein feed but if you have carnivores you might be depending on how big they are. This gives you some wriggle room. This is a product of the fish not the wastes. Solids wastes are assumed to be removed, if not increase everything by 50% and keep an eye out of elevated nitrite levels.
This is only related to ammonia removal from your bio filtration in liters per hour, what ever that might be. It also assumes a maximum water temperature of 31 degrees Celsius and a maximum pH of 7.4. If your water temperature is higher, most Australian species will not survive or they will stress. If your pH goes above 7.4 at that temperature, the free ammonia (the toxic stuff) will be above the limit of 0.025mg/L.
Media surface area
This is any media you like, which can be kaldnes type, gravel or what ever you like to obtain the required square meters needed for ammonia removal. We have assumed 1m2 of media has the surface area to remove or process 0.3 grams of total ammonia. For example if you have gravel it may have a surface area of 120m2/m3, and you have 20kg of fish you will need 40m2 or 300 liters of gravel to process the ammonia. We have also assumed your biofilter is only 50% efficient and you have no passive nitrification on tank walls etc, to give you more wriggle room.
Oxygen by fish and bio filter
This is the amount of oxygen your fish and your bio filter (including autotrophic and heterotropic bacteria) will use in grams per hour, every hour of the day. This assumes you are feeding throughout the day so if feeding in one go, the oxygen demand will be higher for about 4 hours then be back to normal demand.
Time until stress or death
This is when your system stops working from power failure, pump failure etc. I think it is important to know how long you have to get your back up systems in place and running. With 20kg in our tanks we turned the aeration off in the tank alone and we had something like 6 hours until oxygen levels were at critical. This assumes you have oxygen saturation in your water at 31 Celsius of 7.4mg/liter at zero altitude.
Watts for aeration
This is to help you size the amount of watts required by an air pump using a suitably rated air stone (medium pore). Normally we recommend 20 liters per minute per 1000 liters of water but you can see at 20kg/m3 that would be border line.
This is the amount of liters per day to be taken out and replaced to maintain Nitrate levels at a minimum of 80mg/L. This is purely recirculating without any plant integration. For example at 15kg/m3 density we would need to exchange at least 113 liters (7.5%) per day but at 50kg/m3 we would need to exchange 350 liters (35%) per day to maintain that 80mg/L Nitrate in the fish system.
Number of plants
This is worked out of protein use after conversion by a plant. This is a very “soft” number because the type of plant eg: tomato, will use more than a lettuce, but it gives you an idea of the number of plants to support.
Is yet another “soft” number. We have used a mega joule of 22 per m2 for an average in Australia over the last year for around the Tropic of Capricorn. While the solar exposure is a little higher than the south, some seasons have more than others. So we had to pick a number to reflect the worst case scenario. This is worked on a m2 basis with approximately 17 plants per square meter.
- Keep your temperatures below 31 degrees Celsius
- Keep the pH below 7.4
- Ammonia below 1mg/L
- Feed rates below 1.2% of the biomass or the grams as noted
- Flow rate no lower than noted, higher is better. Higher flow rates will reduce the aeration required but you can adjust the flow of both as needed
- Aeration in the fish tank as noted (important to match airstone flows)
- Water exchange required to keep the nitrate levels down (note in recirc they can go much higher than 80 but will cause issues with nitrite)
- Surface area minimums for bio filtration, more is better
- Protein in feed below 50% which should be easy to do
- Have a backup plan. Generators are the best choice and test it often.
A final note
You will notice the 50 kg per 1000 liters requires a flow rate of 2.4 times per hour to keep the ammonia in check, but also it requires about 70 watts of aeration which is close to 80 liters per minute. You may have all seen our round air stones which are rated at 20 liters per minute, in this case you would need 4 of them in a 1000 liter tank to keep up with the oxygen demand of the fish and bio. I can assure you the tank will be more like a spa bath than a fish tank and the turbulence will destroy the flow in the tank and macerate the solids into such small bits you will never get them out.
I suggest if you are starting out, take a look at the green in the table, get a feel for that for at least a year or until you get some fish harvested and step into the orange if you feel inclined. Completely avoid the red section of the table because it is quite difficult to sustain and completely inefficient way of growing fish. Much easier to grab another 1000 liter tank if you need more fish.
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