← Glossary · 🦐 Inverts & cleanup
Copper
Copper is a metal that is lethal to all invertebrates and corals even in trace amounts — present in many fish medications — so it must never enter a reef tank.
This is why sick fish are treated with copper in a separate quarantine tank, never the display, and why RODI water and invert-safe products matter. Copper can leach from old tap water and some equipment too.