Copper is commonly used in aquariums to treat external parasitic infections, like Ich and velvet. It is crucial to closely monitor copper used in aquariums to ensure the correct dose has been applied, especially in volumes of water that are unknown and with copper-sensitive species. This allows the treatment to be effective against the parasites, but still safe for the fish. Some types of copper (e.g. ionic copper) requires redosing to maintain a therapeutic concentration and should be monitored regularly. Water should also be tested after treatment to confirm a zero ppm copper concentration before reintroduction of any invertebrates. Copper can also contaminate some tap water and should be avoided or treated before use in systems that contain desirable invertebrates.