mg/L
Iron
What is it
Iron (Fe) is an essential trace nutrient for plant life. In water it occurs mainly as iron II (Fe²⁺, ferrous, soluble and readily available to plants) and iron III (Fe³⁺, ferric, insoluble and unusable).
In oxygen-rich aquarium water with a neutral or alkaline pH, valuable Fe²⁺ quickly oxidizes to Fe³⁺ and precipitates out of the water column. Commercial fertilizers therefore use chelators—molecules such as EDTA, DTPA, and EDDHA that shield the iron atom chemically—to keep it soluble and plant-available over time.
Iron does not normally affect fish health at aquarium concentrations, but it is the most important and most frequently dosed trace element for growing aquatic plants.
Why it matters
Iron is indispensable for chlorophyll synthesis, the green pigment that enables photosynthesis. It is also crucial to the intense coloration of red plants such as Macrandra, Alternanthera, and Ludwigia: without sufficient iron, a red plant becomes washed out or green.
Iron is immobile within the plant. Unlike nitrogen, a plant cannot move iron from old leaves to new ones, so deficiency symptoms ALWAYS appear first on new shoots and newly formed leaves.
Iron availability is heavily influenced by pH: basic chelators such as EDTA break down quickly above pH 7.0, leaving iron unusable within hours. At pH > 7.5, use fertilizers containing DTPA- or EDDHA-chelated iron.
Ideal ranges
Interactions with other parameters
Iron is highly unstable. UV light or a filter overloaded with peat or carbon can break chelators or absorb iron. It rapidly precipitates with phosphate when they are dosed together at high concentrations.
| Tank type | Min | Max | Unit |
|---|---|---|---|
| Tropical community | 0.05 | 0.2 | mg/L |
| Planted high-tech | 0.2 | 0.5 | mg/L |
| Planted low-tech | 0.05 | 0.1 | mg/L |
| Shrimp tank | 0.01 | 0.05 | mg/L |
Out of range: what happens
Deficiency: the unmistakable sign is iron chlorosis on new leaves. New shoots emerge very pale green, nearly white or transparent yellow, while the leaf veins may remain darker. Red plants completely lose their vivid coloration and become washed-out orange or green.
Excess: chronic overdosing of iron and trace elements can cause plant-tissue toxicity, including pinpoint necrosis, and is associated with rapid brush algae (BBA) and filamentous algae growth, particularly when CO2 or filtration is not optimal.
In tanks with sensitive shrimp, including Taiwan Bee and Pinto, heavy doses of liquid fertilizers containing iron and complex chelators can cause long-term moulting problems. Keep dosing minimal.
Common Myths
- Red plants need enormous amounts of iron; in reality, they need very strong light and low nitrates, while enormous iron doses create only toxicity.
- A zero iron test means deficiency; in reality, chelators almost always stop the kit from detecting it, so do not rely on the liquid test.
Measurement and adjustment
How to measure
Liquid-reagent iron testing is notoriously difficult in aquariums. Many kits measure only free iron, which is zero because the iron is chelated, or cannot break the bonds of stronger chelators such as DTPA/EDDHA, so they report zero despite heavy dosing.
For that reason, experienced aquascapers rarely test iron. They instead read the plants: chlorosis on apical leaves is a much more accurate biological indicator than the chemical test.
If you do test, for example with JBL or Sera, test about one hour after fertilizer dosing to estimate the peak concentration. Testing the next day will almost always return zero.
How to adjust
To raise it when apical leaves are pale, increase your trace-fertilizer dose slightly. In densely planted tanks with strong light and CO2, daily trace dosing prevents excessive peaks and provides a consistent supply.
Choose the right fertilizer: if the aquarium is at pH 7.5 or higher, common in non-CO2 tanks or African-cichlid tanks planted with Anubias, use root tablets, whose roots absorb Fe independently of water pH, or seek liquid fertilizers formulated for alkaline water.
To lower it, stop liquid fertilization and make a couple of large water changes (50%). There is no effective direct method to filter out excess iron without removing everything else as well.
Pro Tips
- Instead of testing water, read the plants: if new shoots emerge white-transparent or pale yellow with green veins, iron is lacking. If they emerge green and become smaller, calcium or CO2 is lacking.
- In tanks with pH > 7.0, standard chelators (EDTA) fail. Use fertilizers with strong chelators such as DTPA or EDDHA.