Inputs · Nutrients and supplements
Iron is the nutrient aquaponic systems run short of most often, because fish feed carries far less than vegetables need. It is added as a chelate: iron bound to an organic molecule that keeps it dissolved, since plain iron drops out of solution at the near-neutral pH aquaponics runs at. Most sources aim for about 2 mg/L.1, 2
SRAC 454 says the iron that comes from fish feed is not enough for hydroponic vegetables and must be topped up with chelated iron. FAO adds that new systems often show iron deficiency in their first 2 to 3 months, because young plants need iron and feed has little. The sign is yellowing between the veins of the newest leaves first (iron does not move within the plant), then the whole leaf turning pale yellow and eventually white with dead patches and distorted edges (FAO). pH makes it worse: SRAC 454 says iron, manganese, copper, zinc and boron become less available above pH 7.0, and the UF/IFAS guide HS1252 says iron and manganese deficiency may appear at pH 7.5.
Target: about 2 mg/L of iron (SRAC 454; FAO says it is regularly added to reach about 2 mg/L, and the Fort Valley State extension slides give 2 mg/L). FAO's start-up advice is 1 to 2 mg/L for the first 3 months and again whenever deficiency shows; elsewhere it suggests 5 millilitres per square metre of grow bed when deficiency is suspected. The review by Kasozi and others reports practitioners dosing 2 mg/L every 2 to 3 weeks. Which chelate: SRAC 454 calls Fe-DTPA the best because it stays soluble at pH 7.0, and says Fe-EDTA, common in hydroponics, is less stable at pH 7.0 and needs frequent replenishing. Kasozi and others give working ranges: EDTA is stable from pH 4.0 to 6.3, DTPA from 4.0 to 7.0 (recommended for systems at pH 6.0 to 7.0), and EDDHA from 4.0 to 9.0; they recommend EDDHA or HBED once pH rises above 6.5 and call EDDHA a good choice when starting a system. Foliar spray is a second route: SRAC 454 and HS1252 both say iron can be sprayed on leaves, though the review notes foliar results vary and chelates penetrate leaves slowly.
UF/IFAS (SL219) says much of Florida's well water is alkaline, pH 7.6 to 8.5, because it has been in contact with the limestone (calcium carbonate) aquifer. Systems topped up with that water may sit above pH 7, where SRAC 454 says iron is less available and where Kasozi and others recommend the EDDHA or HBED chelates over EDTA.
Too much iron harms fish: the Kasozi review says iron can form flocs on the gills, cites a safe dissolved level of 1.69 mg/L and an optimum of 0.35 to 1.0 mg/L total iron, and concludes iron in an aquaponic system should stay below 1.7 mg/L, which is lower than the 2 mg/L target in SRAC 454 and FAO (see gaps). The same review says adding iron again and again without measuring it is dangerous. FAO says a larger dose does not harm the system but can stain tanks and pipes. Chelates break down in light (one study the review cites found Fe-DTPA's half-life in strong sunlight was eight minutes), so keep stock solutions out of daylight and shade the water. Kasozi and others also describe Fe-EDTA as slightly toxic to plants.
Read the label for the chelate name. SRAC 454 favours DTPA at pH 7.0; Kasozi and others favour EDDHA (or HBED) above pH 6.5; EDTA suits only slightly acid water. FAO says chelated iron, also sold as sequestered iron, can be bought as a powder from agricultural suppliers. The label's iron percentage is what you need to work out a dose to reach your mg/L target.
Restated in our own words from these documents, written 2026-11-02. Doses are the sources' figures; test your water before and after.