Is Hard Water or Tap Water Better for a Fish Tank?

In many parts of the United States and the world in general, hard water and tap water are the same thing. Hard water just means that a given sample of water contains dissolved minerals, specifically various forms of calcium and bicarbonates. In terms of aquarium fish, this can be good or bad depending on the specific species of fish, since some fish prefer hard water and some prefer soft.

  1. Know Thy Fish

    • All fish have adapted to the water chemistry of their home waters. For example, fish from the Amazon River prefer soft, acidic water that resembles the water they come from. This includes many South American aquarium fish, like tetras, small cichlids and various catfish. In contrast, fish from the African Great Lakes, like the African cichlids, prefer hard, alkaline water. Always research the fish you plan on keeping to determine if it needs hard water, soft water or does not have a strong preference so long as you avoid extremes.

    Checking Your Parameters

    • You need to keep a few things in mind before you try to "correct" your tap water. Many common aquarium fish can adapt to a wide range of water hardness, and changing it may stress them out. With this in mind, you can procure water test kits from pet shops for both hardness and pH. You should test for both of these parameters since they are related to each other. In tap water and aquarium conditions, hard water usually has a higher or alkaline pH and soft water usually has an acidic pH. You need to dechlorinate all tap water before using it in an aquarium, regardless of hardness and pH.

    Going Soft

    • If you're sure you want to soften your water, you have several options. First, you can use reverse osmosis to purify you water. RO is a process that uses semipermeable membranes and water pressure to remove almost all of the minerals from water. This softens aquarium water. Additionally, you can add decorations like Malaysian driftwood or African mopani wood -- available at pet shops as decorations -- which add to the look of the tank while softening and acidifying the water. A bag of peat in your aquarium filter can have similar results. Keep in mind that commercial water softeners sold for homes do not produce safe water for aquariums; these softeners substitute sodium salts for calcium, making the water dangerously salty for freshwater fish.

    Hardening Your Water

    • For some fish, like livebearers and the aforementioned African cichlids, you may need to harden your aquarium water. Generally, it is easier to harden water than to soften it. You can raise the hardness of your water by decorating the aquarium with calcium-rich materials like coral sand and various forms of limestone. Additionally, pet shops sell African cichlid salts which raise the pH and hardness of aquarium water.