The number of fish necessary in an aquaponics system depends on the size and species of fish and how many plants you’re planning on growing. Of course, the size of the tank you’re using matters as well. It’s probably better to look at the inches of fish per gallon ratio, and calculate that using the expected adult (harvesting) size of your chosen fish species. You can also calculate the number of plants needed to consume the likely volume of available nutrients. If your numbers differ widely, it’s probably best to err on the lower end, at least until you have some experience under your belt.
In general, 1” of fish per 1 gal of water is used in hobby pond-keeping. In intensive production environments, especially where fish are heavily fed and harvested and the aeration is good, you can certainly push it further, so long as there is adequate room for the fish to move around and the water chemistry remains optimal. In an intensive production case, you may decide to allow no more than 1lb of fish weight per 5-7 gallons of water. For example, if you had a 500 gallon tank, and your fish were in the 2lb range, you would need to keep your population below 250 fish. An easy way to do that is by harvesting the big ones and replacing them with fingerlings who can then use the extra room to grow! If fish production is not your focus and you’re planning on keeping the minimum to support your plant passion, then you can go the other direction and calculate that you need approximately 1lb of fish for every square foot of grow bed surface area. For example, if you’re planning on setting up 3 beds with a surface area of 2’ x 4’ , that’s 24 feet of grow area, so ideally you’ll have about 24 lbs of fish. That could mean a couple hundred little goldfish or about 10-12 mature tilapia. Of course, we’re talking mature sizes here. When your plants are small, it’s a good idea to have small fish, too. You want the waste production of the fish to match the nutritional needs of the plants, with neither overwhelming the other.
