Iron (Fe)

The ideal iron level in a planted freshwater aquarium is 0.05–0.2 mg/L, with anything above 1.0 mg/L counting as critical. Iron doesn't harm fish or shrimp at concentrations a tank will realistically reach — the whole point of the number is plant colour and health. Because dissolved iron falls out of solution within hours to days, the target is best read as "always a trace present, never allowed to accumulate" rather than a level you set once and forget.

What it is

Iron is the most important micronutrient in planted-tank fertilisation. Plants use it to build chlorophyll precursors and to colour stem-plant leaves anywhere from deep green to vivid red. Dissolved iron oxidises and precipitates out of the water within hours to days unless held in solution by a chelator (EDTA, DTPA, gluconate).

Why it matters

A planted tank with no detectable iron grows yellow-green and dull. Most red stem plants (Rotala rotundifolia, Ludwigia palustris, Ammania) need a sustained background iron presence to express their colour at all — bright light alone won't do it.

But iron is the easiest micronutrient to over-dose; above 1 mg/L it can stain silicone and trigger green-water blooms. The hobby target is "always present, never accumulating".

The chelator makes or breaks it

Whether your dose survives to reach a leaf depends almost entirely on the chelator and your pH. EDTA-chelated iron — the cheap kind in most all-in-one micros — releases its grip above about pH 6.5 and the iron precipitates before the plants get it, which is why an EDTA product can read zero a day after dosing in hard, alkaline water. DTPA holds up to roughly pH 7.5, and gluconate (the "Fe gluconate" in many premium micros) is a fast-release form that's available immediately but gone within a day. In a tank running pH 7+, an EDTA-only product is close to useless; reach for DTPA or dose gluconate little and often. Match the chelator to your water before you conclude the dose is too small.

The pH ceilings, side by side:

Chelator	Holds iron up to	Character
EDTA	~pH 6.5	cheap, the standard in all-in-one micros; fades fast in alkaline water
DTPA	~pH 7.5	the safe default for most tap water
EDDHA	~pH 9	near-unconditional grip; tints the water faintly red at full dose
Gluconate	— (fast release)	available to plants immediately, gone within a day

There's a nutrient interaction worth knowing too: high phosphate reacts with iron to form insoluble iron phosphate, so a tank pushing PO₄ at the top of its band will burn through iron faster than the dosing chart predicts. If your iron reads low despite regular dosing and everything else checks out, an aggressive phosphate level is a common hidden sink.

How to test

JBL Eisen, Sera Fe — both rated for the 0.05–1.0 mg/L hobby band. Strip tests are too coarse. Test 24 hours after dosing; iron drops fast, so the value you see depends heavily on when you measured relative to the last dose.

That timing point is the whole game with iron. Test an hour after dosing and you'll read a spike that's already on its way out; test the morning before the next dose and you'll read the trough. Neither is wrong, but pick one and stick to it, or the readings won't be comparable. One more quirk: most hobby kits measure total dissolved iron and read chelated and free iron alike, but the colour develops slowly at the bottom of the band — give it the full stated wait and read in good light, because the difference between 0.05 and 0.1 mg/L is a couple of pale shades. Logging a run of readings at a fixed point in the dose cycle — the water-test log guide shows a workable format — tells you far more than any single number: you're looking for a stable trace, not a headline figure.

A flat zero isn't automatically a deficiency verdict either. On a gluconate-heavy routine the iron is taken up or precipitated within hours, so the water column tests empty even while the plants are eating well. When the vial and the leaves disagree, believe the leaves.

What high and low look like

Below 0.05 mg/L: chlorosis on new growth, red plants reverting to green. Above 0.5 mg/L: green-water risk, particularly under bright light; cyanobacteria sometimes blooms. Above 1 mg/L: silicone staining starts.

The direction of the yellowing is the tell. Iron is immobile in the plant, so a deficiency shows on the newest leaves first — pale, yellow-between-the-veins tops while the older leaves stay green. (Contrast a nitrogen shortage, which yellows the old leaves as the plant robs them to feed new growth.) Reds fading to orange then green is the same story in a species that was banking iron for pigment. Livestock, again, show nothing at any level short of the extreme.

Hobby folklore also files staghorn algae under "too much iron". The evidence is thin — staghorn tracks overall organic load at least as closely — but if it turned up the week after you doubled the micros, halving them again is a free experiment.

Water column or roots?

Not every plant drinks its iron from the water. Heavy root feeders — Cryptocoryne, Echinodorus swords, most bulb plants — take micronutrients through their roots, and an iron-rich root tab pushed under a struggling sword often fixes in a fortnight what months of column dosing didn't touch. Aquasoils arrive loaded with iron for roughly their first year, which is why a fresh soil tank shows rich colour on no dosing at all and then fades quietly as the substrate exhausts. The sorting rule: pale rosette plants over healthy stems means feed the substrate; pale stems too means feed the column.

How to fix

Dose a chelated iron product daily or every other day rather than weekly — the chelator releases iron slowly enough that small frequent doses give better stable concentration than one large weekly hit. Tropica Specialised, Aqua Rebell Mikro Basic Eisen, Easy-Life ProFito. The dosing calculator handles the mL-per-litre arithmetic.

Over-dosing is the rare micronutrient problem that mostly fixes itself: skip a few doses and the iron oxidises out on its own within days, or bring it down faster with a water change. In a low-tech tank the whole equation eases off — slower growth means lower demand, so a light weekly dose usually covers it; the planted-without-CO₂ guide covers how far you can dial the ferts back when the tank isn't racing. If you dose comprehensive micros, remember iron travels with the other trace elements (manganese, boron, zinc), so a product that keeps iron in range generally keeps the rest fed as well.