Why dry-salt dosing
Bottled liquid fertilizers are convenient. They're also expensive, mostly water, and limited to whatever profile the manufacturer baked in. For a 200-litre planted tank running heavy lighting and CO₂, a year of commercial liquids easily clears €200. A year of the same nutrients in dry salts costs about €15 and lasts three years.
What you need is rarely complicated:
- KNO₃ - potassium nitrate - provides NO₃ (nitrogen) and some K (potassium)
- KH₂PO₄ - monopotassium phosphate - provides PO₄ (phosphate) and a little K
- K₂SO₄ - potassium sulphate - provides only K, used to top up potassium
- A trace-element mix – CSM+B, TNC Trace, or your local equivalent – for iron and micros
The three popular profiles
Estimative Index (EI)
Created by Tom Barr in the early 2000s. The premise: dose nutrients in deliberate excess, do a large weekly water change (50%) to reset, and never have to worry about nutrient limitation. Weekly targets, roughly:
| Nutrient | Target (mg/L) |
|---|---|
| NO₃ (nitrate) | 20 – 30 |
| PO₄ (phosphate) | 2 – 5 |
| K (potassium) | 20 – 40 |
PPS-Pro
Lean dosing. Smaller daily doses, smaller water changes (10–20% weekly), aimed at "just enough" - tracks the same nutrients but targets levels plants will mostly consume between water changes.
EI Low / "Modest EI"
Halfway between the two. EI doses cut by 50%, water changes ~30% weekly. The setting most established planted tanks settle at after a couple of years of fiddling.
When to dose and when not to
Three principles cover most cases:
- Dose before lights-on - plants take up most nutrients in the first 4 hours of the photoperiod.
- Skip the water-change day - anything dosed gets siphoned out a few hours later.
- Separate macros from micros by ~12 hours if you dose iron-heavy traces; phosphate + iron can co-precipitate.
The Redfield ratio (without the chemistry exam)
The original Redfield ratio comes from marine plankton and is way too specific for what we do. In planted tanks, the practical version is: keep N : P at roughly 10:1 by mass. Too much phosphate relative to nitrogen - i.e., N : P below 5:1 - correlates with green-spot algae on slow-growing leaves and the front glass.