If you wanted to take care of aquarium fish that aren’t for beginners, you should consider water hardness. There are species who can’t survive in a climate that’s unlike their native waters. You should test their hard water to make sure they have a happy home.

hard water - aquarium

They stare into your soul a little, but that’s fine in the long run.

Hard Water: What is It?

The concept of hard water might confuse some of our readers, but the idea is pretty simple, depending on whether you’re thinking, technically or functionally. On a technical standpoint, hardness measures if water leaves a strong mineral residue during its evaporations. On a functional standpoint, hardness measures how much calcium and magnesium is in the water. The water hardness in an aquarium has a strong correlation to high PH.


Your test kit can measure either general hardness (GH) or carbonate hardness (KH), but remember that they can’t do both. Your GH involves the total of your dissolved solids, while your KH looks at how many carbonate minerals dissolve in the water. It won’t include other factors like organic compounds or sodium salts. You’re mostly looking at your GH, but your KH comes into play when you have coral or specific aquarium plans.


You collect a sample of the aquarium water in a test tube provided in the kit. Afterward, based on the instructions, you add the number of drops of chemical (reagent) that changes the color of water, depending on its hardness. There will be a color chart you can use to match the correct colors to the corresponding water hardness.


You can also collect water in a small bin to dip a strip into it for a strip test. You should never dip a strip right into your aquarium. When the appropriate amount of time passes, you can compare your strip to the color chart to determine water hardness.


You’ll rely on two different units for general hardness (GH) and carbonate hardness (KH). Those are ppm (parts per million), which can also be written as ppmGH or ppmKH by whether its general hardness or carbonate hardness. Your alternative is degrees of hardness that uses their written as dGH or dKH.

If you’re ever in fear about whether you know your stuff, make sure to talk to your local fish shop and do your research. You’ll have a happy home for your fish in no time!