Fact: The water in an aquarium or pond should never be replaced all at once with soft water

If your tap water is softened using an ion-exchange water softener (such as the ‘Mooiwater’ system), the calcium and magnesium ions are replaced by sodium ions. This reduces the German hardness (dH) to 0dH. However, this value is actually never mentioned on the various websites about the quality of aquarium or pond water. When it comes to aquarium or pond water, the focus is always on KH (Karbonat Härte, German for carbonate hardness) and GH (Gesamt Härte, German for total hardness). Both of these values change when using a water softener.

Although the correct levels of these KH and GH values may vary depending on the species of fish you keep, it is always a safe option not to alter the water composition. If your fish are thriving in the water you have always used, then when filling your aquarium or pond you can bypass Just give it a quarter-turn. You’ll then have your (unsoftened) tap water back straight away. It’s a piece of cake, and perhaps you and your fish will feel more comfortable that way. Don’t forget to put the bypass back in place after filling the tank!

See also: Is rainwater better for my garden than soft or hard tap water?