Iron (Fe)
The micronutrient that decides whether your red plants are red. Tiny target band, fast turnover, easy to over-dose.
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".
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.
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.
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.
