Reef-Safe vs Copper Treatments: Why Copper Kills Invertebrates
Copper is the workhorse treatment for marine fish parasites, but it is lethal to invertebrates and corals and is destabilised by live rock. Learn the therapeutic copper level for fish, why carbonate rock binds it, and why parasite treatment belongs in a separate hospital tank.
The therapeutic window for fish
Ionic copper (Cu2+) treats parasites such as Cryptocaryon and Amyloodinium by killing their free-swimming stages. UF/IFAS gives a therapeutic level of 0.15 to 0.20 mg/L free copper; Merck lists 0.18 to 0.2 mg/L for up to 3 weeks. The level is reached gradually over 2 to 3 days and must be tested at least twice a day, because the window between effective and toxic is narrow.
Why copper is not reef-safe
Most invertebrates are highly sensitive to copper and will not survive therapeutic levels. They must be removed before treatment and not returned until copper reads 0.01 mg/L or less, ideally zero. Merck states plainly that copper is extremely toxic to invertebrates and plants. Corals, crustaceans, snails and other inverts are all at risk.
Why live rock destabilises copper
Calcium and magnesium carbonate materials — dolomite, oyster shell and coral skeleton — dissolve and complex (bind) with copper, pulling it out of solution. This makes dosing in a display with live rock or substrate unreliable. Worse, a drop in pH releases previously bound copper back into solution, spiking free copper and the risk of toxicity.
Safe practice
- Remove all inverts, corals and plants before any copper treatment.
- Treat in a bare hospital tank without calcareous rock or substrate.
- Raise copper gradually over 2 to 3 days to 0.15 to 0.20 mg/L and test twice daily.
- Keep pH stable — a pH drop spikes free copper.
- Monitor ammonia and nitrite, as copper disrupts the biofilter.
- Do not return inverts until copper reads 0.01 mg/L or less, ideally zero.
Sources: ask.ifas.ufl.edu (UF/IFAS FA165); www.merckvetmanual.com ; ask.ifas.ufl.edu (UF/IFAS FA164)