Copper Test Kits
High levels of copper are toxic to aquatic life. Copper is an effective treatment for parasites, and you need to use copper test kits whenever you use copper-based medications to make sure levels don't get too high. (Use a quarantine tank to prevent problems in your main tank.) Once treatment is complete, use activated carbon to filter any copper out. Copper should not be used or evident in aquariums with invertebrates or planted aquariums.
Copper is sometimes present in tap water - use copper test kits when setting up your tank or doing water changes from a new water source.