By Tom Lang
We have been seeing more and more homes installing water softeners and then having problems with their aquariums. In order to soften a home’s tap water, many water softeners work by exchanging minerals (mostly calcium and magnesium) with sodium. While this is good for your pipes, shower doors and washing machines, it’s not great for fish.
Aquarium water needs to contain a balance of electrolytes (solutes that break down into ions when dissolved in water) in order for fish to be able to regulate their internal homeostasis. Freshwater fish, especially, require some calcium and magnesium as well as other minerals in their water for this osmoregulation since they continuously absorb water into their bodies though their skin and continuously out through dilute urine produced by their efficient kidneys. Electrolytes are lost from the body during urination and if these same ions are not present in the surrounding water, then the fish is unable to achieve internal electrolyte balance by active transport through its gills and this rapidly leads to death.
So it is not so much the presence of sodium that causes fish problems in softened water, but rather it’s the lack of calcium, magnesium and other important minerals that fish need to survive in an aqueous environment.
In a freshwater planted aquarium, a balance of minerals is also important, however it is interesting to note that plants may do fine for quite a while in softened water even as the fish perish. This is due to a hydrophyte’s (fully submerged aquatic plant) tendency to retain minerals in its tissues since it does need to work to retain water like a terrestrial plant that loses water through transpiration. Calcium and magnesium are considered “secondary” nutrients because plants require them in smaller quantities than nitrogen, phosphorus, and potassium. Often, aquarists with planted tanks will also add plant nutrients that contain enough calcium, magnesium and other minerals for the plants, but not in enough quantity to provide for the far greater needs of the fish.
Finally, using softened water in a saltwater fish or reef aquarium creates other issues. Again, the lack of calcium and magnesium may require supplementation to bring them back into line with natural seawater. While saltwater fish are drinking seawater rather than absorbing it through their skin like freshwater fish, they still must regulate their internal electrolyte balance. It is necessary to add calcium and magnesium to artificial saltwater made with softened water if tests indicate levels below natural seawater.