Copper cations constitute an essential ingredient of a wide range of aquatic agents acting against protozoa, algae and fungi used in both freshwater and marine aquariums. For the long-term baths against protozoa, it is required to maintain the concentration of copper cations above 0,2 mg/l. However, an actual concentration of copper cations depends not only on the quantity of an added agent but also such factors like water content and the type of a substrate are of great importance. In hard water, especially in seawater, the effectiveness of agents is decreased due to the precipitation of some copper ions in the form of sparingly soluble compounds. It is therefore required to supplement this element to maintain its required concentration. Fish can tolerate significantly higher copper levels in comparison with protozoa that are combated. Even though it should be taken into consideration that too high concentration of copper may affect adversely and be toxic to fish as well. Long-term use of copper supplements under favourable conditions may contribute to the release of copper ions from formerly insoluble solids. In such a situation, lack of control may result in elevated copper content presenting a risk for an aquarium. In the case of vertebrates, animals more primitive than fish, copper is toxic in quantities used against protozoa. It is not allowed to use copper supplements in saltwater aquariums containing sea anemones, snails and starfish. The tap water can contain small quantities of copper dissolved from plumbing and other installation components. For aquariums inhabited by especially vulnerable animals, it is also advisable to control the copper content in used freshwater.