GH / KH remineralizer
How much remineralizer to add to RO or distilled water to hit a target GH (and KH if the product raises both).
Why this exists
Reverse osmosis water comes out at near zero TDS, zero GH, zero KH. Fish, shrimp, and plants all need some mineral content to live, so pure RO is a starting point, not a finished product. You have to put minerals back in before adding livestock or substrate.
Product picks
Three products cover most use cases.
Salty Shrimp GH+ is the standard for shrimp tanks (caridina especially) because it raises GH cleanly without touching KH. Soil-buffered shrimp tanks want zero KH so the substrate can hold pH down where caridina like it. GH+ is the only sane option there. The tub's figure is 3 g per 20 L for about 6 dGH.
Salty Shrimp GH/KH+ is for community tanks or fish that prefer harder, buffered water. Raises both GH and KH together at a fixed ratio, about 1 dKH for every 2 dGH. The tub's figure is 2 g per 10 L for roughly 6 dGH and 3 dKH.
Seachem Equilibrium is the easiest to find in physical stores and works well for planted tanks. GH only, no KH. Seachem's figure is 16 g per 80 L to raise GH by 1 meq/L, which is 3 dGH. If you also need KH, add baking soda (sodium bicarbonate) separately: 0.03 g per liter raises KH by 1 dKH, so 3 g in 100 L. It's a small amount, weigh it rather than eyeballing a spoon.
Test, don't trust
The factors in this calculator come from the current product instructions. Specific gravity of the salts varies a little batch to batch and the math is only as accurate as the dose. Measure with a GH test kit after the first mix.
The baking soda number is chemistry rather than a label: 1 dKH is 17.86 mg/L of calcium carbonate equivalent, and the same alkalinity from sodium bicarbonate is 30 mg/L.