Many ornamental fish, invertebrates and also most aquatic plants in the aquarium hobby like soft and acidic water with a pH below 7, a low carbonate hardness and a rather low general hardness. In reality, this is not always easy to achieve in the aquarium if you take medium-hard German tap water as your starting point. To make aquarium water softer, many aquarists use a reverse osmosis system, which produces fully demineralized water. However, this alone must not be used in the aquarium, as important minerals are missing. One way to bring these into the aquarium water is to bring the water back to the desired parameters using water-hardening salt. Another method is blending it with tap water. In this article, we show how you can easily calculate the mixing ratio.

Under what conditions should you blend water?

If you want to use your tap water in addition to the demineralized water, it should meet certain requirements. The starting values of drinking water are not always usable for a particular aquarium setup. You can look up the exact parameters of your source water online in the form of the drinking water analysis from your local water supplier. One example is the so-called silicate value. If the drinking water is too heavily loaded here, there is a likelihood that persistent diatoms will spread in the aquarium. This is not necessarily the consequence of elevated silicate values in the source water. However, if you have permanent problems with diatoms, you should look for the cause and check the silicate value of your drinking water for greatly excessive concentrations.
If this is the case, it would be better to use pure water from a reverse osmosis system with a downstream mixed-bed resin filter, which reliably eliminates silicate. The water is then hardened with a suitable mineral salt.
But other excessive concentrations of substances such as nitrate, phosphate, chlorine or heavy metals are not always tolerable for the aquarium either. For planted aquariums, an unfavorable calcium-magnesium ratio could also be an exclusion criterion for using tap water.

If tap water is generally suitable for your individual needs, you can use it for blending with demineralized water. This enriches the zero-hardness water from the reverse osmosis system or full deionizer with enough minerals (especially calcium and magnesium) and carbonates.

Calculating the blending proportions

To determine the proportions of tap water and demineralized water, we use the so-called cross rule. First, though, you should decide what specific target value you want to reach for either the general hardness (GH) or the carbonate hardness (KH). Unfortunately, when blending, you cannot set both parameters independently of each other, because in tap water they already have a fixed ratio to each other and are merely diluted by adding demineralized water.
The cross rule is best explained with the following diagram:

For this you need the values of the tap water, those of the demineralized water (abbreviated here as RO water, i.e. reverse osmosis water) and of course the desired target value. Now you always calculate in the direction of the arrow: first you subtract the target value from the value of the tap water and enter the result at the bottom right as "RO water proportion". Then you subtract the target value from the value of the RO water and note the result under "Tap water proportion" (important: ignore the sign!). At the bottom you now have the two proportions in which you have to mix the liquids to arrive at the desired target result. For simplicity, you can take these numbers as liters.

Let's take an example: our drinking water has a general hardness of 18, and we want to reach a GH of 6. The hardness of our demineralized water is 0. We enter these values in the corresponding places at the top and in the middle of the arrow diagram. Now we calculate 18 - 6 = 12 and enter the result under "RO water proportion". Next we calculate 0 - 6 = -6. We ignore the sign of the result and enter the 6 as the value under "Tap water proportion". Consequently, we need 6 liters of tap water and 12 liters of RO water to arrive at the desired value.

Cross rule with rainwater

If, to blend with hard tap water, you use not demineralized water but soft water from another source (such as rainwater), the cross rule can be applied here as well. Simply replace the corresponding RO water positions on the right-hand side with rainwater. The calculation is done in the same way. Since rainwater or other natural soft water is not completely free of minerals, the values for KH and GH will very likely not be zero. In the calculations, this is an important difference compared to fully demineralized water.

Fine-tuning the water parameters

As mentioned above, when blending tap water with demineralized water we cannot set the general hardness (GH) and the acid-binding capacity, or carbonate hardness (KH), independently of each other. Both parameters are fixed by the individual properties of the tap water. We can only dilute these values; the ratio remains the same. With a suitable hardening salt, however, this problem can certainly be worked around. Suppose you have mixed your water according to the cross rule to lower the carbonate hardness, but the general hardness has become too low as a result. Then you can bring this value up to the desired level with a hardening salt that affects only or mainly the GH. Or the other way around: blending has made the carbonate hardness too low. Then you can set it back to the desired value with a KH plus salt, without this significantly affecting the general hardness.

Calcium-magnesium ratio

Especially in planted aquariums, a particular ratio of calcium to magnesium is important; ratios of around 3:1 are commonly used here. Unfortunately, hard tap water often contains hardly any magnesium but is very rich in calcium. Blending with softer water lowers the general hardness, but the ratio of Ca to Mg always stays the same. To achieve a more favorable ratio, you can use Epsom salt in your water treatment to add magnesium.
Note: Adding Epsom salt also raises the general hardness to a certain extent. You can avoid exceeding the desired GH value by aiming in advance, when blending the water, for a correspondingly lower general hardness as your target. Only by adding Epsom salt and the hardening that comes with it is the desired general hardness finally reached.