Can you replace all the water in your aquarium or pond with soft water in one go?

In this blog, you’ll find out why softened tap water isn’t necessarily suitable for your aquarium or pond, exactly what changes in the water, and how you can still use your usual tap water without having to do without your water softener. The short answer: no, don’t replace all the water with softened water in one go. Fish are sensitive to factors other than those addressed by a water softener.

Does a water softener affect your aquarium or pond? 

A water softener It alters the composition of your tap water, and this is relevant as soon as you have an aquarium or a pond. Fish are not sensitive to the parameter that is key to a water softener (dH), but they are sensitive to two other parameters that change as a result: KH and GH.

Which parameters are important for aquarium or pond water?

So KH and GH are the values to keep an eye on, not dH. But what exactly do those two measure, and why does that make a difference to your fish? Hardness sounds like a single concept, but dH, GH and KH do not measure the same thing.

• dH (German Hardness) is the measure used by water companies and water softeners. It indicates how much calcium and magnesium there is in tap water. Curious about the Water hardness in your local authority area?
• GH (Gesamt Härte, total hardness) In the aquarium world, this is effectively the same as dH: the amount of dissolved calcium and magnesium. Fish and plants need these minerals for, amongst other things, cellular function, and in the case of crustaceans and shellfish, for the formation of their skeletons.
• KH (Karbonat Härte, carbonate hardness) measures something else: the amount of carbonates and bicarbonates in the water. This determines the water’s buffering capacity, or in other words, how stable the pH level remains. If the KH is low, the pH can fluctuate sharply and suddenly, and this is often more harmful to fish than an abnormal hardness level in itself.

A water softener exchanges calcium and magnesium ions for sodium ions. As a result, the dH drops to 0, and the GH and KH of the water change accordingly. This makes no difference to your washing machine or coffee machine. However, for your aquarium or pond, it does alter the mineral balance to which your fish are accustomed, even though it is exactly the same tap water that you drink yourself.

How do you use the bypass when filling your aquarium or pond?

Every Mooiwater water softener has a built-in bypass. If you’re going to fill your aquarium or pond, turn the bypass a quarter turn. You’ll immediately have access to your old, unsoftened tap water again, without having to disconnect the water softener. Make sure to turn the bypass back once you’ve finished filling, otherwise the rest of your house will also be supplied with hard water. You can find a full explanation of the bypass, including an instructional video, on our bypass page.