Goldfish can tolerate a wide range of water chemistry, including both soft and hard water. However, soft water can mean different things to a chemist and an aquarium enthusiast. Some water softeners soften water in a way that produces water unsuitable for goldfish. Understand what kind of water you have helps you determine its safety for goldfish.
Soft Vs. Hard Water
Water's hardness consists of a measurement of the concentration of certain minerals in the water. Specifically, hard water has high levels of dissolved calcium and bicarbonates. In nature, hard water always has a high pH and high alkalinity. Conversely, soft water always has low and acidic pH low levels of dissolved calcium and low alkalinity. Hard water can cause problems for domestic uses. For example, soap does not dissolve as well in hard water, making chores such as washing clothes and dishes more difficult.
Reverse osmosis produces soft water suitable for goldfish. This process uses a semi-permeable membrane and water pressure to remove all dissolved minerals from water, softening it. While reverse osmosis can be expensive, it produces water that will not harm a goldfish. However, RO water is overkill; goldfish usually do not need water treatment beyond dechlorination.
Commercial Water Softeners
While RO-made soft water will have no effect on goldfish, some soft water can. Many domestic water softeners aim to produce water suitable for household chores. This form of softening water basically swaps calcium and bicarbonates out for other minerals, usually sodium-based minerals. While this water has no effect on humans, it can prove deadly to fish. Unlike reverse osmosis, this produces water that is still mineral-rich, but saturated with an unusual combination of minerals, including salt, that fish would not encounter in nature.
Since domestic water softeners soften water by replacing calcium and other minerals with sodium, the effects are similar to putting a goldfish in saltwater. Goldfish are stenohaline, meaning they can only tolerate water with a relatively narrow range of salinity. When you put a freshwater fish in saltwater, the fish begins to lose water from their cells to the surrounding environment. This can kill the fish depending on how much sodium is in the water.
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