The acids continuously produced in an aquarium are neutralised and the carbonate hardness is stabilised. In addition, phosphates are precipitated by the formation of insoluble compounds. Due to the uniform and gentle addition of lime water, a sudden increase in the pH value is effectively prevented. The stirring mechanism ensures constant saturation of the lime water and the low number of revolutions prevents the direct entry of calcium hydroxide into the aquarium water. The supply of water (de-ionised water) to the lime water mixer can ideally be achieved by means of an automatic refiller.