Aquarium: Hardening the Water
By Aram Schneider Published Updated
Translated from German with AI assistance and reviewed by our editors.
Every aquarist strives for the perfect water parameters for their animals and aquatic plants. However, the tap water you use dictates many parameters that sometimes do not suit your stocking well. If that is the case, appropriate water treatment is advisable. Here we show you how best to bring aquarium water to the values you want.
Why not use tap water?
In many cases, the local tap water can be used for an aquarium. Local drinking water suppliers usually offer a water analysis on their websites. These analyses are very precise and extremely comprehensive. So right there, aquarists can read up on whether the drinking water is suitable for their particular purpose. However, tap water in Germany is often too hard for the needs of the aquarium's inhabitants, which often prefer soft and acidic water parameters. This applies to many South American ornamental fish and aquarium plants, but also to various animals and plants from Asia and Africa. In these cases, softer water with a lower carbonate hardness and general hardness should be used. But other parameters of the source water can also be unfavorable. A high concentration of silicate, for example, can cause persistent diatom problems in the aquarium. Depending on the setup, high nutrient concentrations of phosphate or nitrate are also undesirable, for example with sensitive high-grade shrimp. There are various well-proven ways to remove all of these substances from the water.
Demineralizing water
First of all, the goal is to get the tap water as free of all elements as possible. In the aquarium hobby, a so-called reverse osmosis (RO) system is often used for this. Here, the water is forced under high pressure through a fine membrane that retains all substances with a particle size larger than water molecules. Another popular option is the use of a full deionizer based on the principle of ion exchange. Here, both cations and anions are removed from the water by different columns and exchanged for either H+ or OH- ions. These two then react to form pure water (H2O). A third option, which may be suitable for small nano aquariums, is simply to buy distilled water (for example at a drugstore or hardware store) in ready-made canisters. Make sure that the water is also approved for use in aquariums. This is often noted on the packaging. Another option is to use purified rainwater - please only in regions with little air pollution! The rainwater is collected with gutters (if you want to keep invertebrates, you should use plastic gutters rather than copper ones) and then filtered through activated carbon for at least 24 hours.
Hardening
Fully demineralized water should not be used in its pure form in the aquarium. It lacks vital minerals for the animals, plants and microorganisms in the biotope, and there is a risk of over-acidification and a pH crash. For this reason, water-hardening salts are used for the fresh water. They enrich the water with all the minerals and trace elements that are important for the biological system and add a certain degree of carbonate hardness (KH) and general hardness (GH). Often, some important plant nutrients such as potassium are also contained in the hardening salt. Of relevance for planted aquariums: parameters such as calcium and magnesium are often already in a very good ratio of about 3:1, so that elaborate readjustment with Epsom salt is no longer necessary. How many degrees of general hardness and/or carbonate hardness you want to set your aquarium water to depends primarily on the requirements of the inhabitants and aquatic plants you want to keep.
There are various hardening salts that different manufacturers offer:
- Salts that only raise the KH.
- Hardening salts for raising the GH in a targeted way.
- Salts that raise both the KH and the GH in a balanced ratio.
Mineral salt mixtures that raise both carbonate hardness and general hardness (often recognizable by the label "GH/KH+") are particularly easy to handle, since the preset ratio of GH to KH (often 1:0.5) produces good and suitable results in most cases. For typical soft water values, you can harden the aquarium water to a KH of 2 and a GH of 4 this way. For keeping shrimp from habitats with neutral water, such as the popular Neocaridina, and also for some planted aquariums, a somewhat higher general hardness is recommended to avoid deficiency symptoms. Values of around KH 3 and GH 6 would be more suitable here.
If, on the other hand, you want to set the carbonate hardness and the general hardness individually, it is advisable to work with two separate salts for hardening KH and GH respectively. This makes it possible, for example, to achieve very low carbonate hardness with a higher general hardness, which is sometimes necessary for breeding certain ornamental fish and invertebrates. Especially for soft water shrimp, there are special hardening salts (usually recognizable by the label "GH+") that raise the carbonate hardness only at a ratio of 0.06 to 1.0 relative to the general hardness and thus have practically no effect on the KH. In this case you would not need any KH+ additive at all.
Calculating the amount of hardening salt
An example: SaltyShrimp - Shrimp Mineral GH/KH+ raises the GH to approx. 6 and the KH to approx. 3 when 2 g are added per 10 liters. This corresponds to a conductivity of about 300 µS.
Of course, all manufacturers of water-hardening salts state in the product description what amount is needed to harden the water intended for the aquarium to a certain level of GH or KH. In some cases the hardness is given directly (in °dGH or °dKH), in others a specific conductivity in microsiemens (µS). If you want to do the math yourself, you can also use the Flowgrow Nutrient Calculator. Its database contains many different salts from various manufacturers, so determining the correct amount is child's play. What matters is that you, as an aquarist, know the net volume of your aquarium water as precisely as possible (that is, the gross volume minus all elements that displace water, such as decoration and substrate).
Adding the hardening salt
The hardening salts commonly used in the aquarium hobby differ in solubility. Some salt mixtures dissolve very easily in water, and mixing up fresh water for the aquarium takes only a few seconds. Others need a little more time and possibly some help. To improve solubility, it may be necessary, for example, to raise the water temperature, add CO2 or stir vigorously.
In these cases, it is best to mix the water in a separate container, which you equip if necessary with suitable equipment such as a heater, pump or CO2 supply device to make the process as effective as possible. Once the salt has completely dissolved, the treated water can be used directly in the aquarium. Any residue still left in the container is generally added to the aquarium as well; these more stubborn components will continue to dissolve in peace over time right where they are.
Adding the demineralized water and the hardening salt directly may seem convenient, but it is only recommended in exceptional cases. Mixing the water directly in the aquarium can be too great a shock, especially for sensitive plants and animal inhabitants.
Switching your aquarium water from tap water to treated water
If you have so far run your aquarium on pure tap water and would now prefer to switch to remineralized pure water, you should proceed with caution and patience. Replacing all the water in the aquarium at once can be a shock for the inhabitants. This applies above all if the water values of tap water and treated water differ greatly. The sensitivity of the animal stock also plays a role here. A slow adjustment is recommended. For example, it makes sense to simply switch the fresh water to remineralized RO water during a water change and use it for your weekly water change. Through the regular exchange, over a few weeks the values in the aquarium will move closer to the values you want to achieve with the hardening salt. For the inhabitants, this is a very gentle way of getting used to the new conditions.
Even if it sometimes takes a little while, your ornamental fish, aquatic plants and shrimp will be very happy about the improved water conditions, which shows in more intense colors, vital growth, a greater willingness to breed and improved health.