What should I consider when purchasing a fish tank for my aquaponics system?
- If you have the space, most aquaculturalists will tell you that a round fish tank is best. That said, we know plenty of aquaponic gardeners growing in rectangular and square tanks that do just fine. Just make sure any tank you consider does NOT have hollow pockets in the bottom or any other place where solid waste can accumulate.
- In general, bigger fish tanks tend to be more stable, and it is best to shoot for at least 90- 100 gallons if you are growing edible fish to plate size.
- You should stock your tank at our recommended "Rule of Thumb" of 1 pound of fish for every 5 - 10 gallons of water. An "average" amount of fish consumed per year according to the FAO is 37 pounds. A 200 gallon tank will produce about that each year.
- You will want to have at least a 1:1 ratio of 12" deep grow bed volume to fish tank volume. Much less than this will not provide adequate filtration for your fish. You can go all the way to a 3:1 grow bed : fish tank volume, but be sure to add a sump tank or two at that point so the water level in your fish tank does not go too low during the flood cycle.
What is a sump tank?
A "sump" is an area where liquid run-off accumulates. For example, in a basement the sump is where water that seeps in drains to. In a boat the sump is what is also known as the "bilge". In an aquaponics system the sump sits at a lower point than the grow beds and is where the grow beds drain to. "CHOP", "CHOP2" (Constant Height One Pump) and CHIFTPIST (Constant Height In Fish Tank Pump In Sump Tank) designs employ a sump tank. The reason is to both maintain a constant height in the fish tank, which is best for the fish, and to add more water to the system overall, thus creating more system stability with regard to pH and temperature.
How large of a sump tank do I need?
Since the sump tank is where the water flows in and out of the grow beds you want your sump tank to be big enough to handle the situation when all of your grow beds are full at once. You need to add the total water volume of your grow beds, then subtract the displacement effect of the grow media (for Hydroton we've found that 55% of the water is displaced, and 62% with 3/4" gravel).
Once you have figured this out, then you also need to calculate the minimum amount of water must remain in the sump tank to cover the pump and insure that it does not run dry and burn out. We use 3", but your pump intake height may be different than ours. This volume should be added to the grow bed water volume calculation above and this total is the minimum volume you need for your sump tank.
You can easily connect two sump tanks with a simple bridge siphon and create essentially a single, larger sump tank this way.
What else do I need to consider?
Pay close attention to the height and width of the sump tank to make sure that it will fit, unobstructed, under your grow beds. If you have a dirt floor then you can sink the sump into the ground, so height is less of a consideration.