TankDosing

How to make a fertilizer stock solution

A stock solution turns weighing powder every other day into pouring a few mL. The trick is to scale the per-dose grams up to the whole bottle, and to keep the bottle grams and the tank dose clearly separate.

The #1 mix-up: the grams in a stock recipe go into the bottle once. What goes into the tank is the mL per dose. Our calculator shows a blue STOCK SOLUTION banner when it is showing bottle grams.

The math

  1. Work out grams per dose for your tank (dry-dose mode).
  2. Decide bottle size and mL per dose. Doses per bottle = bottle mL ÷ mL per dose.
  3. Grams in the bottle = grams per dose × doses per bottle.

Example: 1.85 g KNO₃ per dose, 500 mL bottle, 10 mL per dose → 50 doses → 92.6 g KNO₃ in the bottle.

Watch solubility

Salts only dissolve so far. Approximate limits at room temperature: KNO₃ ~316 g/L, KH₂PO₄ ~220 g/L, K₂SO₄ ~110 g/L (the one that most often refuses to dissolve), MgSO₄·7H₂O ~700 g/L. Stay below about 70 % of the limit, or crystals form in a cold room. If the calculator warns, use a bigger bottle or more mL per dose.

Step by step

  1. Use distilled or RO water if you can; tap water is OK for macros.
  2. Weigh each salt on a 0.01 g scale.
  3. Fill the bottle about ¾, add salts, shake until clear (warm water helps), top up to the exact volume.
  4. Macro bottle: KNO₃, KH₂PO₄, K₂SO₄ and usually MgSO₄. Micro bottle: CSM+B only.
  5. Label with contents, date, and "x mL per dose for y L".
  6. Keep micros cool and dark; remake if mold or cloudiness appears.

Ready-made recipe

Open the calculator in stock mode for a 40 US gal EI tank, 500 mL bottle, 10 mL per dose, or a 60 L lean-EI tank. Use the "suggest mL" button to make each bottle last about four weeks.

Liquid carbon and other additives

Keep glutaraldehyde products and other additives out of your fertilizer bottles. Dose them separately per their own label.

FAQ

Can I put MgSO4 in the macro bottle?

Usually yes. Many keepers put Epsom salt in the macro bottle; keep it out of the iron (micro) bottle.

How long does a stock solution last?

Macro solutions are stable for months if kept closed. Trace (micro) solutions are best used within a month or two and stored cool and dark.
Where to buy · Retailer links coming soon.

Open the dosing calculatorMore guides

Sources

  1. r/PlantedTank: kit instructions vs 500 mL bottles
  2. r/PlantedTank: stock solution questions (K2SO4, MgSO4, Excel)
  3. PPS-Pro: example 500 mL bottle recipe
  4. Potassium nitrate: solubility
  5. Potassium sulfate: solubility
  6. Monopotassium phosphate: solubility
  7. Magnesium sulfate: solubility

Written in our own words from the sources above. Numbers are educational starting points; check product labels and your water.