Remineralizing RO Water

How to add minerals back to pure RO water, the difference between GH and KH, and target hardness ranges for soft-water fish and dwarf shrimp.

Why RO water needs remineralizing

Reverse osmosis produces nearly mineral-free water with a general hardness of zero and no buffering capacity, so it is unsafe for most livestock on its own. Remineralizing restores the dissolved minerals fish and invertebrates require for osmoregulation and gives the water a controlled, repeatable parameter set.

GH versus KH

General hardness (GH) reflects dissolved calcium and magnesium. Carbonate hardness (KH), measured as carbonates and bicarbonates, provides alkalinity and the buffering capacity that resists pH change. Both are expressed in German degrees (dGH, dKH) or as ppm of calcium carbonate, where one German degree equals about 17.85 ppm as CaCO3.

Choosing a target

Targets depend on the intended species. The USGS classification labels water as soft at 0 to 60 ppm CaCO3, moderately hard at 61 to 120 ppm, hard at 121 to 180 ppm, and very hard at 181 ppm and above. Soft-water and blackwater fish suit the lower end, while many livebearers prefer harder water.

Targets for dwarf shrimp

Most keepers of soft-water Caridina species use pure RO water remineralized with a GH-boosting product, often dosing baking soda separately if any carbonate hardness is required. Caridina such as Crystal and Bee types favour soft, low-KH, acidic water around 5 degrees general hardness, 0 to 2 dKH and pH 6.0 to 6.8. Neocaridina are more robust and do well across medium to harder water at pH 6.5 to 8.0.

Remineralizing products

Dedicated shrimp salts raise GH to a target with each dose. A general-purpose salt typically produces about pH 7.0 to 7.5 at 6 to 8 degrees general hardness, while a softer Bee-type salt yields a more acidic balance around pH 6.0 to 6.5 and roughly 5 degrees hardness. Some products raise GH only; carbonate buffering is added separately when needed.

Stability matters most

Sensitive shrimp are affected by sudden changes in carbonate salt concentration and dislike KH swings of more than about three degrees in a short period. Mix and measure new water to the same parameters as the tank before each water change to keep GH and KH consistent.