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In this video, the well-known German aquascaper, Oliver Knott explains how to test various types of hardscape stones for lime. As a means of determination, he drizzles oak extract on various well-known aquarium stones such as Ohko, lava, Seiryu and holey rocks. Oak extract is available as an accessory in aquarium shops. In this experiment we'll make use of its acidic properties. If you sprinkle some of the liquid on calcareous stones, it will cause a chemical reaction, perceptible as clearly visible gas bubbles on the stone. This reaction does not occur with non-calcareous stones.
Since the majority of aquatic plants, tropical ornamental fish and pet shrimp prefer soft, acidic water in the tank they inhabit, any aquarium keeper should actively try to avoid everything that potentially hardens the water. The following point is important, though: Calcareous stones tend to significantly harden the water if a CO2 system is used. If there is no carbon fertilization via CO2, the influence on the water parameters may be smaller. The reason for this is the small amount of carbonic acid that forms when carbon dioxide (CO2) is introduced. This acid reacts with the lime embedded in the stones and breaks down the solid limestone. During this reaction, ionized calcium (Ca2+) forms, which increases the total hardness (GH) of the water, and carbonates (HCO3-), which increase the carbonate hardness (KH) in the aquarium.
If you want to use hardening stones in your planted aquarium or aquascape for aesthetic reasons and still keep the water as soft and acidic as possible, there are several means to reach this goal. As Oliver Knott explains, regular big water changes with soft freshwater are a remedy. Moreover, he recommends the use of active aquarium substrate - aka soil -, which can help remove minerals from the water and to lower the pH as well as the carbonate hardness and the total hardness.
Another solution is offered by the company ADA. Softenizers can provide for soft and acidic water in continuous operation in the aquarium. This special glassware is filled with ion exchange resin, which continuously reacts with the water and reduces the hardness.
By the way: in our guide "Hardscape", we have listed all types of aquarium stones from our online shop, sorted from neutral to hardening.