How much potassium does a planted tank need?
Potassium is the third macro nutrient after nitrogen and phosphorus. It is hard to test reliably at home, which is why most dosing plans add it on a schedule and let water changes cap the level.
Typical potassium targets
| Method | K added per week | NO₃ per week | PO₄ per week |
|---|---|---|---|
| Estimative Index (EI) | 30 ppm | 22.5 ppm | 3 ppm |
| Lean EI | 15 ppm | 12 ppm | 1.5 ppm |
| PPS-Pro style (daily) | 9.3 ppm | 7 ppm | 0.7 ppm |
| Estimative-style low-tech (weekly) | 5 ppm | 5 ppm | 0.5 ppm |
EI guidance is commonly given as about 20–40 ppm potassium a week for high-light CO₂ tanks; PPS-Pro uses much smaller daily amounts. These are educational starting points from the method authors, not measured requirements for your tank.
Where potassium comes from
By weight, KNO₃ is 38.7 % K, KH₂PO₄ is 28.7 % K and K₂SO₄ is 44.9 % K (from IUPAC atomic weights). So EI's KNO₃ for 22.5 ppm nitrate already brings about 14.2 ppm K a week, and KH₂PO₄ for 3 ppm phosphate adds 1.2 ppm. Add K₂SO₄ only for the rest. The calculator does this automatically: it tops up with K₂SO₄ only what is still short.
Grams of K₂SO₄
1 g of K₂SO₄ in 100 L adds 4.49 ppm K. For EI in common tanks, per dose (3× a week): 10 gal 0.41 g (410 mg) · 20 gal 0.82 g (820 mg) · 40 gal 1.64 g · 55 gal 2.25 g · 75 gal 3.07 g.
Signs of too little or too much
Potassium shortage typically shows as pinholes and yellowing edges on older leaves. The calculator warns if a plan would build potassium above 100 ppm at your water-change rate. See nutrient deficiency signs.