Practical Fishkeeping Diploma - Week 1, Day 2


Here's the second of a five-part series of lessons to guide new aquarists through the basics of fishkeeping.

Week 1, Day 2: Water hardness

Hardness refers to the amount of mineral content in water, and in aquaria we are specifically interested in carbonate hardness (KH) and general hardness (GH).

Carbonate hardness

Carbonate hardness, or KH, is determined by the amount of calcium carbonate dissolved in to water. It is sometimes referred to as the alkalinity of water (not to be confused with alkaline as in the pH value, which is a different use of the same word) as its presence helps to stabilise pH. Water lacking adequate carbonate hardness will become unstable, and the pH may fluctuate dangerously. Water high in calcium carbonate often gives itself away in the form of limescale that can form on taps, in kettles, and around the top of aquaria.

KH levels in a tank can be reduced by:

Using water that lacks calcium carbonate (such as RO or rainwater) for waterchanges.

Carbon dioxide injection or respiration by fish and plants.

Biological filter activity.

KH levels can be raised by: 

Using water that contains high calcium carbonate (some tapwater around the UK) for waterchanges.

Making a calcium carbonate solution and adding it to the aquarium.

General hardness

General hardness, or GH, is determined in aquaria by the amount of calcium and magnesium salts present. GH is what we refer to when we talk of hard or soft water for fish.

General hardness is usually measured in dGH (sometimes expressed as just H) or degrees of general hardness. Each degree equates to 17.86mg/l of calcium and magnesium ions.

Aquarium hardness levels are rated according to the following scale:

0 to 4dGH – very soft water

4 to 8dGH – soft water

8 to 12dGH – medium hard water

12 to 18dGH – fairly hard water

18 to 30dGH – hard water

Over 30dGH – extremely hard water

GH levels in a tank can be raised by:

Using water with a high general hardness at source (some tapwater around the UK).

Adding commercially available GH boosters that contain calcium and magnesium ions.

Adding a solution of Epsom salt, magnesium sulfate.

GH levels in a tank can be reduced by:

Using with a low general hardness at source (RO and rainwater).

Using a water softener resin.

Click here for next lesson: Water quality

How to gain your diploma: Once all the course modules and revision pages have all been posted online, we will open a link to a website that allows you to take your free online exam. If you pass the exam, you will digitally receive your very own Fishkeeping Diploma, to show that you have successfully completed the course, and which is yours to display on the wall near your aquarium, hang in your fish house — or keep somewhere safe where you can take it out and just look at it from time to time.

Note: The Fishkeeping Diploma is not a formal or accredited qualification and should not be confused with the type of diploma presented by colleges, universities and other educational establishments.

Further reading from our sponsors

Treating common aquarium problems 

Routine aquarium care

Troubleshooting for green cloudy water