Hard Water and Aquarium Fish

Fish often adapt more easily than expected

Guppies
Robert Pickett/Corbis Documentary/Getty Images

For many people, hard water is a fact of life. Going to great lengths to soften aquarium water for the sake of the fish within, however, doesn't have to be. Before you wage war with the specific parameters of your tank's water, take a moment to consider if the fish you have selected truly need softer conditions or if they can adapt to what flows from the faucet.

A Hard Water Primer

The hardness of water refers to the amount of dissolved minerals in it, and it's measured in two ways: General Hardness (GH) and Carbonate Hardness (KH), also referred to as Alkalinity. The former measures magnesium and calcium, while the latter measures carbonate and bicarbonate ions. When measuring the hard water for fish, it's referred to as either degrees of hardness (dH) or as parts per million (ppm). A degree of General Hardness (dH) is defined as 10 mg/L CaO, which is equivalent to 17.85 ppm.

  • When the dH is 0 to 6 and the ppm is 0 to 100, the water is soft or very soft.
  • When the water's dH is 6 to 25 and the ppm is 101 to 449, it's slightly hard to hard.
  • When dH is 30 or more and the ppm is 450 or more, the water is considered "liquid rock," or very hard.

Water's KH is related to the aquarium's pH level. The higher the KH measurement, the less the pH of the aquarium will fluctuate—and that's best for the fish.

Choosing Hard Water Fish

Here's the good news: Unless you've invested in specific tropical species that absolutely must live in soft water, such as a wild-caught Discus, your fish will likely adapt to the hardness of the local water in its aquarium.

Even if the research you do on fish accurately states the original native habitat for a species of fish, it's possible the fish you bring home from your local pet shop wasn’t born or raised in that environment. In fact, since most fish species are now commercially bred, odds are it was raised in water that leans toward the hard alkaline side.

However, you can bypass the whole issue of whether your fish will thrive in hard water by simply choosing a hard water fish species. These include:

  • Livebearers such as Guppies, Mollies, Platies, and Swordtails
  • Paradise Fish
  • African and some Central American Cichlids
  • Brackish fish like Archers, Monos, and Scats
Altolamprologus compressiceps Boulenger, 1898
African cichlid. Javier Millán Photography / Getty Images

Softening or Hardening the Water

There are a number of ways you can soften hard water if necessary, including:

  • Water-softening pillows
  • Peat
  • Driftwood

If your chosen species of fish truly must have soft water, consider changing water sources instead of using expensive ongoing water treatments. Using Reverse Osmosis (RO) water to blend in is one option, as is using a combination of tap and distilled water. Some industrious aquarium owners have been known to collect rainwater, which is naturally soft and acidic.

On the other hand, if you find that your water is far too soft, there are ways to harden it, too, including:

  • Crushed coral or oyster shell
  • Limestone
  • Buffer additives

If you use any sort of extras to harden or soften the water, ensure that they've been cleaned thoroughly, so it doesn't do more harm than good.

Ask the Experts About Hard Water Fish

When purchasing fish, check with your local pet shop to find out what the hardness and pH levels are for their tanks. You might be surprised to find that most of their tanks are filled with hard, neutral to alkaline water, even though they are keeping fish that are supposedly soft water species.

It seems counterintuitive to keep fish in the "wrong" type of water, but these fish were captive-bred in hard water conditions. Therefore, it makes sense to keep them in water that is similar to what they were raised in.