Choosing Fish For a Hard Water Aquarium

If you have hard water, choosing fish for your aquarium might seem challenging. Some hobbyists go to great length to soften their water to suite certain types of aquarium fish, but it is not always necessary to do so. Unless you are interested in breeding wild Discus or keeping some other wild-caught fish that come from soft water habitats, it may not be a concern at all. There are plenty of species that tolerate hard water and some that even prefer it, plus many aquarium fish that are found in soft water in the wild are commercially bred today in moderately hard to hard water.

What Makes Water Hard?

Water hardness is determined by dissolved minerals. General Hardness (GH) measures primarily calcium and magnesium, while Carbonate Hardness (KH), also known as alkalinity, measures carbonates and bicarbonates.  These compounds also act as buffers that help prevent fluctuations in pH, which is important to fish! For aquarium purposes, General Hardness and Carbonate Hardness are measured in degrees (dH and dKH respectively), or parts per million (ppm).





0 - 4°

0 – 75 ppm

Moderately Hard

4° – 8.5°

75 – 150 ppm


8.5° - 17°

150 – 300 ppm

Very Hard


300+ ppm

Hard Water Fish

Most aquarium fish will adapt to moderately hard to hard water provided they are acclimated properly.  Ask your local aquarium shop about the hardness and pH in their tanks and compare it to the water in your aquarium. The greater the difference, the more gradually fish should be acclimated when you bring them home.

Another approach, if you have hard water, is to choose fish that are accustomed to it to begin with.  Here are some fish that live in hard water in the wild:

  • African Rift Lake Cichlids (Lakes Malawi, Tanganyika and Victoria) Chromides
  • Livebearers – Guppies, mollies, platies, swordtails X Ray Tetras
  • Most Central American Cichlids Glass Fish
  • Certain types of Rainbowfish and Killifish Most Freshwater Gobies
  • Brackish Water Fish Paradise Fish
Aqueon Choosing A Fish For A Hard Water Aquarium

Wild Caught Fish

While most freshwater aquarium fish today are commercially bred, there are still a few wild-caught species available that should be kept in soft water. If you are interested in keeping or even breeding these fish, it is best to try and match their natural habitat as closely as possible. Here are some ways to safely soften hard water and maintain stable conditions over the long term:

  • Use reverse osmosis (RO) or deionized (DI) water to create the desired hardness and pH.
  • Blend RO or DI water with your tap water to achieve the desired hardness and pH.
  • Add aquarium-safe peat moss or peat pellets to your filter.
  • Decorate the aquarium with driftwood to help lower hardness and pH. (In extremely hard water, this method works best when used along with one or more of the options above.)

Having hard water does not have to be an obstacle to keeping cool fish. With so many different varieties to choose from, you are bound to find some real beauties that do well in hard water. The best approach is to know your aquarium’s water chemistry, and always research fish before purchasing them.

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