Softener-Treated Water in Aquariums: Ion Exchange Chemistry

Household ion-exchange softeners swap calcium and magnesium for sodium or potassium. Why the result is not natural soft water, how it affects fish, and which sources work instead.

How a household softener works

A domestic water softener is an ion-exchange unit. Hard water passes through a bed of resin beads or natural zeolite that holds loosely bound sodium ions. Calcium and magnesium, the divalent ions that make up hardness, bind to the resin more strongly and displace the sodium, which is released into the water (North Dakota State University Extension; University of Nebraska–Lincoln Extension). Wikipedia describes the exchange as replacing each calcium or magnesium ion with twice the number of monovalent sodium or potassium ions. When the resin is saturated it is regenerated with a concentrated salt brine, and the rinse water carrying the removed calcium and magnesium is discharged to the drain.

Hardness in softener literature is often given in grains per gallon (gpg); NDSU Extension gives 1 gpg as about 17 mg/L. Potassium chloride can be used for regeneration instead of sodium chloride, in which case potassium rather than sodium enters the water; NDSU notes it binds more strongly to the resin and lowers exchange efficiency.

What changes in the water

General hardness (GH) falls towards zero, because calcium and magnesium are removed.
Carbonate hardness (KH) stays essentially the same. Only cations are exchanged; Wikipedia notes that anions such as bicarbonate, chloride and sulfate remain in the softened water.
Sodium rises. Both NDSU and UNL Extension give about 8 mg/L of sodium added for every grain per gallon of hardness removed, so water of 10 gpg (about 170 mg/L) leaves the softener with roughly 80 mg/L of extra sodium. Wikipedia cites a study that found a mean of 278 mg/L sodium in softened water.
pH is not lowered directly, and dissolved gases such as CO2 are not removed (SRAC Publication 4607).
Many other substances pass through unchanged: UNL lists bacteria, hydrogen sulfide, silt, lead, nitrate, pesticides and many other organic and inorganic compounds.
Iron and manganese are partly removed: NDSU gives up to 10 ppm for a standard softener, while University of Georgia Extension gives up to 5 ppm of combined iron and manganese.

Why it is unsuitable for many fish

Softened water is low in hardness but chemically unlike the soft waters that many aquarium fish come from. Wikipedia gives the Rio Negro, a classic blackwater river, a pH of about 5.1, a conductivity of about 9 µS/cm, and roughly 0.2 mg/L calcium and 0.4 mg/L sodium. Water from a softener fed with moderately hard, alkaline supply water keeps its full bicarbonate load and pH and carries tens to hundreds of milligrams of sodium per litre, so it is neither acidic nor low in dissolved solids.

Calcium matters to fish physiology. The Southern Regional Aquaculture Center (SRAC Publication 464) explains that calcium and magnesium are needed for bone and scale formation, blood clotting and other metabolic reactions, and that dissolved calcium reduces the leakage of sodium and potassium from fish blood into the water; in low-calcium water fish lose substantial amounts of these salts and spend energy reabsorbing them. SRAC also notes that water with moderate to high bicarbonate alkalinity but less than 25 mg/L hardness can swing above pH 9 during intense photosynthesis, because sodium and potassium carbonates are more soluble than the calcium carbonates that would otherwise precipitate. Extension services in North Dakota and Nebraska advise against using softened water for house plants, lawns and gardens because of its sodium content.

Alternatives

Unsoftened supply: many installations already have a cold-water line to the kitchen tap that bypasses the softener for drinking and cooking (UNL, NDSU). Water drawn before the softener has the original mineral content and can be treated like ordinary tap water.
Reverse osmosis (RO): Wikipedia notes that RO removes dissolved ions including sodium, calcium, magnesium and potassium, and that RO water is used in marine and reef aquariums to prepare artificial seawater and replace evaporation. A household unit discharges roughly 3 to 25 litres of reject water per litre produced, and RO water needs remineralising before freshwater fish are kept in it.
Blending: SRAC describes mixing softened or RO water with untreated water to reach a target ion concentration. Blending RO water with unsoftened tap water lowers hardness without adding sodium; blending with softened water still carries its sodium.
Rainwater: naturally soft and slightly acidic, but it carries roof and storage contamination risks and also needs remineralising.