How much baking soda to raise KH by 1 degree?
3 g per 100 liters — 0.3 g per 10 L, 1.14 g per 10 gallons, and a level US teaspoon (4.6 g) raises 10 gallons by about 4.1 dKH. That is not a rule of thumb: one dKH is 0.357 milliequivalents per liter, sodium bicarbonate carries one equivalent per mole and weighs 84.007 g per mole. Potassium bicarbonate does the identical job at 3.58 g per 100 L — a heavier molecule, same one equivalent — and is the choice for a planted tank, because what comes along with it is potassium the plants use rather than sodium nothing uses. Whichever one, dissolve it in a jug first and add it to flow; a stocked tank gets no more than 1 dKH a day, and the tool splits anything larger.
| Salt | Per | 10 L | 20 L | 5 gal | 100 L |
|---|---|---|---|---|---|
| Baking soda | 1 dKH | 0.3 g | 0.6 g | 0.57 g | 3 g |
| Potassium bicarbonate | 1 dKH | 0.36 g | 0.72 g | 0.68 g | 3.58 g |
| Gypsum (CaSO₄·2H₂O), calcium only | 1 dGH | 0.31 g | 0.61 g | 0.58 g | 3.07 g |
| Calcium chloride dihydrate, calcium only | 1 dGH | 0.26 g | 0.52 g | 0.5 g | 2.62 g |
| Epsom salt (MgSO₄·7H₂O), magnesium only | 1 dGH | 0.44 g | 0.88 g | 0.83 g | 4.4 g |
| Magnesium chloride hexahydrate, magnesium only | 1 dGH | 0.36 g | 0.73 g | 0.69 g | 3.63 g |
| Gypsum + Epsom, Ca:Mg 3:1 | 1 dGH | 0.2 + 0.16 g | 0.4 + 0.31 g | 0.37 + 0.3 g | 1.98 + 1.56 g |
| Seachem Equilibrium (from its analysis) | 1 dGH | 0.59 g | 1.19 g | 1.12 g | 5.94 g |
Grams to raise that batch by one degree; multiply for your target. From molar masses (1 dGH = 17.85 ppm CaCO₃; 1 dKH = 0.357 meq/L). Equilibrium from Seachem's guaranteed analysis — its label rounds to 6.67 g per 100 L per dGH. Gypsum dissolves slowly: stir it into the whole batch, never a stock.
Every dKH from baking soda arrives with 8.2 ppm of sodium; from potassium bicarbonate, 14 ppm of potassium. Taking a tank from 1 to 4 dKH with baking soda is 25 ppm of sodium that no water change with the same tap water removes, because you will dose the next batch too. It is harmless to fish at that level — and it is the reason a remineralized tank's TDS runs well above its GH and KH would suggest.
How do I raise GH — gypsum, Epsom salt, or a GH booster?
GH is calcium plus magnesium, so raising it means choosing which one, and every salt brings a partner ion. Gypsum (calcium sulfate) is the cheap calcium source at 3.07 g per 100 L per dGH, and it brings 17.1 ppm of sulfate with each degree. Calcium chloride is faster to dissolve at 2.62 g, bringing 12.6 ppm of chloride instead. Epsom salt is the magnesium at 4.4 g per degree (17.1 ppm sulfate). A commercial booster is the same salts pre-blended, usually with potassium: Seachem Equilibrium needs 5.9 g per 100 L per dGH by its analysis and brings 11.3 ppm of potassium per degree — 8× as much potassium as magnesium, which planted tanks like and shrimp keepers generally don't want. The tool shows each dose's calcium, magnesium and counter-ion, so the choice is visible instead of buried in a product name.
What does "3 parts gypsum to 1 part Epsom salt" actually give you?
Not a 3:1 calcium-to-magnesium ratio, which is what the recipe is always presented as. Gypsum is only 23 % calcium by weight and Epsom salt is 10 % magnesium, so 3 g of gypsum to 1 g of Epsom salt is 7.1:1 of the ions. Equal weights are 2.4:1. To actually land on 3:1 — roughly what Seachem Equilibrium is built to and what most shrimp and planted advice means — you weigh out 1.27 g of gypsum per gram of Epsom salt. Nobody's recipe says which "parts" it means, and it matters by a factor of two.
| You want Ca:Mg (by weight) | Weigh gypsum : Epsom |
|---|---|
| 2:1 | 0.85 : 1 |
| 3:1 | 1.27 : 1 |
| 4:1 | 1.69 : 1 |
| 5:1 | 2.12 : 1 |
| 7:1 | 2.97 : 1 |
| "3 parts to 1 part" by weight gives | 7.1:1 Ca:Mg |
Salt weights are the dihydrate (gypsum) and heptahydrate (Epsom) as sold. Calcium chloride to magnesium chloride at 3:1 is 1.32:1 by weight.
Calcium sulfate saturates at about 2.4 g per liter, and temperature barely helps. That is 78 dGH — the strongest gypsum solution water will hold. A one-liter "stock" meant to bring 100 L up 6 dGH would need 18.4 g in it, 7.7× what dissolves; the rest sits on the bottom and the doses that follow are guesses. Weigh gypsum into the whole batch and stir, or use calcium chloride (147 g/mol, dissolves without limit) when you need a concentrate. Epsom salt is fine either way.
How do I remineralize RO water for shrimp?
Neocaridina want GH 6–10 dGH and KH 2–6 dKH (the freshwater parameters chart has the rest). A 20 L bucket of RO to 6 dGH at 3:1 is 2.38 g of gypsum and 1.87 g of Epsom salt — 28 ppm calcium, 9 ppm magnesium, 103 ppm sulfate — plus 1.2 g of baking soda for 2 dKH (16 ppm sodium). The same 6 dGH from Seachem Equilibrium is 7.1 g, with 68 ppm of potassium riding along. Gram scales that read to 0.01 g cost less than one bag of a branded remineralizer; at these quantities a kitchen scale is not accurate enough. Stir until clear, test, then adjust the recipe rather than the bucket — the tool takes a measured GH as "now" and gives the top-up.
How do I lower GH and KH?
By dilution, and by nothing else that can be measured. Nothing you add removes calcium or magnesium; "pH down" and acid buffers destroy KH by converting it to CO₂, which works until you overshoot and the pH falls through the floor, and peat and almond leaves are unquantifiable. So the honest tool is a blend: RO fraction = (tap − target) ÷ (tap − RO), with the RO's own reading honored, because a tired membrane is not zero. Tap water at 12 dGH and a target of 6 is a 50 % blend; the same tap's 8 dKH lands at 4 in that blend, and if you needed it at 2 the blend would have to be 75 % RO and the GH would follow it down to 3 — one blend cannot dilute two things by different amounts, so the tool tells you which parameter set the ratio and where the other one lands. Then, if the blend's GH is now too low, that is a raising job with the salts above.
| Tap water | → 2 dGH | → 4 dGH | → 6 dGH | → 8 dGH |
|---|---|---|---|---|
| 8 dGH | 75 % RO | 50 % RO | 25 % RO | 0 % RO |
| 12 dGH | 83 % RO | 67 % RO | 50 % RO | 33 % RO |
| 16 dGH | 88 % RO | 75 % RO | 63 % RO | 50 % RO |
| 20 dGH | 90 % RO | 80 % RO | 70 % RO | 60 % RO |
Percentage of the mix that is RO (or distilled) water, assuming the RO reads 0. A dash means the tap is already at or below the target.
A tank raised to 6 dKH with tap water at 3 dKH drops to 5.25 dKH at the first 25 % water change, and keeps sliding toward the tap with every one after — the same mixing arithmetic the water change calculator runs for nitrate, in reverse. Dosing the tank is for a one-off correction; the permanent fix is to treat the change water to the target before it goes in, which is what the "a batch of new water" setting sizes. Give the tool your tap reading and it prints what the next change will undo.
How fast can I change GH or KH — and why does KH keep dropping?
Fish and shrimp react to the rate, not the number. The usual advice is no more than 1 dKH or 2 dGH a day in a stocked tank — conventions rather than measured thresholds, and the tool lets you lower either — so taking a tank from 4 to 10 dGH is 3 days of 2 dGH, retesting before each dose. A new batch of water has no livestock in it and gets the whole dose at once. If KH keeps falling between doses, nothing is wrong with the salt: nitrification consumes it continuously — about 0.66 dKH for every 2 ppm of ammonia processed, which the cycling calculator quantifies — and injected CO₂ does not (it moves pH, not KH; the CO₂ calculator covers that relationship). A tank that eats a degree of KH a week needs it in the change water, not in a monthly rescue.