None. That’s the beautiful thing about aquaponics. Aquaponics systems are based on a three way symbiotic relationship between fish, certain types of nitrifying bacteria, and plants. When properly balanced, fish will produce waste (poop) as part of their natural growth process, which is naturally broken down by bacteria into nitrates. These nitrates are pure fertilizer for the crops in an aquaponic system. Think about your compost heap: it’s filled with organic waste (leaves, kitchen scraps, etc.) which breaks down over weeks or months with the help of earthworms and bacteria, and eventually becomes a rich black medium containing heaps of nutrients for your garden. The fish-bacteria-plant cycle is essentially the same, just faster and in an aquatic environment. A good balance of fish to plants and adequate circulation by pumps or other methods will provide a constant supply of high quality nutritious water to your plants. Supplementing with additional fertilizer is actually discouraged, as that can throw your carefully balanced system into chaos.
The symbiotic system isn’t closed, obviously - fish and plants are regularly harvested, so nutrients do need to be reintroduced somewhere, and that happens through feeding the fish. Ideally, fish are fed highly nutritious, highly digestible food, preferably organically sourced. When fish consume this kind of diet, their waste contains all the micronutrients needed by plants to absorb and produce the most nutritious product possible, whether it’s carrots, tomatoes, rutabagas, or lettuce. In fact, if you’re experiencing less than optimal growth from your plants, the key is to add more fish or to improve the quality of their food - definitely NOT to add plant fertilizer!
