Does Water Hardness Affect Fish?

Water hardness has a direct impact on fish health. Water hardness refers to the concentration of metal ions in the water. These usually include calcium and magnesium salts but can also include zinc, iron and other trace metals. Hardness affects everything from the pH level and stability to disease treatments, yet some owners never check the levels of hardness in the water. Knowing the local tap's water chemistry before buying compatible fish and plants is an easy way to improve their health and survival in the aquarium or pond.

  1. Types of Water Hardness

    • Water hardness comes in two types: permanent hardness and alkalinity. These two types combine as a measure of the total (or general) hardness of the water and are usually about equal. The permanent hardness measures nitrates, sulphates, chlorides and other ions that cannot be removed by boiling the water. Alkalinity measures hardness from carbonate and bicarbonate ions. These ions can be removed by boiling. They are the kind of ions that form lime-scale stains on taps. Hard water is usually high in alkalinity, while soft water has low alkalinity.

    PH Levels

    • High mineral content (hard water) leads to high pH water, while low mineral content (soft water) leads to low pH levels. It is impossible to balance the pH levels until the correct level of water hardness or softness is maintained. Once the amount of mineral in the water is controlled, then decreasing the pH is as simple as adding a commercial pH decreaser, and increasing pH can be done by adding more minerals to the water.

    Effects of Water Hardness on Fish

    • Having the wrong water hardness levels in an aquarium can cause stress for the fish inside and can even interfere with the fish's ability to maintain a constant internal fluid concentration (osmoregulation), leading to death. Most pond fish, such as goldfish and koi, thrive with moderate to hard water. Other species may need a different hardness, though, so it is important to check the needs of each species. This is especially important before introducing a new species into a pond or aquarium to avoid conflicting needs. Also check before administering any disease treatments. For example, chloramine-T becomes toxic in soft water.

    Changing Water Hardness

    • The first step to healthy water in an aquarium is testing the water used. Most pet shops or aquarium supply stores sell testing kits that will measure both types of water hardness. To soften hard water and lower the pH, you must separate and remove the mineral. You can do this with a water softener or a reverse osmosis unit. After removing the mineral, you can add a commercial pH decreaser. The easiest way to create hard water from soft is to add a calcium-based rock to the aquarium or pond.