Goldfish are attractive, brightly colored fish that children and adults will love to care for. To keep your goldfish healthy and happy, make sure you have ideal tank conditions, including good quality water. Soft water can harm your goldfish.
Water hardness refers to the measurement of certain minerals--namely calcium, magnesium, iron and zinc--that can build up in a water supply when rainwater travels through rocky areas. Hard water usually has high levels of calcium and magnesium salts. To become potable, hard water is put through a buffering process. This aims to reduce the amounts of calcium and magnesium salts in hard water. Since it has fewer minerals to begin with, soft water is often less buffered than hard water. While this causes no health problems for you, it is not an ideal climate for goldfish.
Water Hardness Indicators
The water hardness scale (see Resources) defines soft water as that with a level of 0 to 75 mg of calcium carbonate per liter of water and hard water as possessing 150 to 300 mg of calcium carbonate per liter of water. The metric measurements are significantly more precise.
Water Hardness and General Fish Health
While each species of fish varies, pond fish such as goldfish and koi prefer a medium to hard water with a level of 100 to 300 mg of calcium carbonate per liter of water. The hard water is preferable because it helps the goldfish better maintain osmoregulation, or the process of keeping a constant internal body fluid concentration. Soft water means the fish have to work harder to keep a constant fluid level, and the increased level of work could mean more bacteria and waste flowing into the fish's body.
Disease and Soft Water
While the osmoregulation is important for the general health of goldfish there are several specific fish illnesses that are greatly affected by water hardness levels. Chloramine-T in particular can become quite dangerous to fish and is significantly more present in soft water
Regulating Water hardness
The first step to maintaining a proper ecosystem is to monitor the water hardness level every two weeks to once a month. If you find a general hardness level that is too low, you can boost hardness levels simply by adding something high in calcium to the water filter, such as a crushed oyster shell. You can also purchase buffering supplies to add directly to your water. Depending on the type of hardness you wish to boost, you can add either a calcium or a magnesium buffer supplement. If your water is too hard you can purchase water softening products at the aquarium supply store to soften it.