FinsFlora

RO/DI remineralization

RO/DI water is close to empty: no calcium, no magnesium, no buffer. Plants need the first two and your pH needs the third. GH (calcium plus magnesium) and KH (carbonate buffer) are set separately, by separate salts, so you can hit each target exactly. Enter the water you're making and the hardness you want; this gives grams of each salt.

Water

Source water is 0 for fresh RO/DI. For a tap/RO blend, enter the blend's measured GH and KH.

Salts

Ratio: 3:1 to 4:1 is common hobby guidance, not settled science.1

Add to 20 L

SaltGramsRaises

What ends up in the water

Ion addedmg/Lmmol/L

A TDS pen reads conductivity and converts it with its own factor, so its number won't match the mg/L total exactly.

How this is calculated 
1 °dGH = 10 mg CaO/L = 10 ÷ 56.077 = 0.178326 mmol/L of Ca + Mg
1 °dKH = the same in alkalinity   = 0.356652 mmol/L HCO3⁻ (bicarbonate is
         monovalent, so twice the moles)
1 °dH  = 17.848 ppm as CaCO3

ΔGH = target − source,  ΔKH = target − source

Split GH between Ca and Mg (r = Ca:Mg by mass):
  molar ratio q = r × 24.305 ÷ 40.078
  n_Mg = n_GH ÷ (1 + q),   n_Ca = n_GH − n_Mg      (mmol/L)

grams = mmol/L × molar mass (g/mol) × litres ÷ 1000
  CaSO4·2H2O 172.164   CaSO4 136.134
  MgSO4·7H2O 246.466   MgSO4 120.361
  KHCO3      100.114   NaHCO3 84.006

CaSO4 and MgSO4 raise GH only; KHCO3 and NaHCO3 raise KH only.

Notes

  1. Ca:Mg ratio. There's no consensus optimum. 3:1 to 4:1 by mass is the usual planted-tank guidance; some shrimp keepers run lower. The molar ratio is shown too, because some sources quote it that way and the two differ by a factor of 1.65.
  2. Gypsum dissolves slowly and not much of it. About 2.4 g per litre at 20 °C. Shake it into the full volume, not a small jug, and give it time; undissolved powder means GH comes in low.
  3. Sulfate and sodium. An all-sulfate GH mix adds sulfate, and baking soda adds sodium. At normal targets both are modest; they're listed so you can see them, not because they're a problem.
  4. Commercial GH boosters (mineral salts sold for shrimp or planted tanks) have their own formulas and often add potassium. This calculator is for the pure salts named above.