How Heavy Metals Affect Aquatic Invertebrates

Shrimp, snails and corals are far more sensitive to metals than fish. That gap is why copper cures a fish tank but destroys an invertebrate tank.

Anyone who keeps shrimp, snails or a reef tank quickly learns to treat copper as poison. The reason is biological: aquatic invertebrates are far more sensitive to dissolved metals than fish are, and this difference in tolerance is one of the most practically important facts in the whole hobby.

The tolerance gap, in numbers

The clearest illustration comes from copper therapy in marine fish. The recommended dose to treat parasites such as Cryptocaryon and Amyloodinium is 0.15 to 0.20 mg/L of free copper, a level marine fish tolerate. Invertebrates do not: most invertebrates are highly sensitive to copper and will not survive a copper treatment, and the guidance is that invertebrates should not be returned until copper is 0.01 mg/L or less, ideally zero. In other words, the safe ceiling for invertebrates is roughly an order of magnitude below the level used to dose fish.

Why invertebrates are so vulnerable

Crustaceans, the group that includes shrimp, crayfish and the planktonic Daphnia, are especially affected. Their sensitivity is so reliable that Daphnia, a small planktonic crustacean, is used as a standard model and indicator organism in aquatic toxicity testing, precisely because it responds so consistently to contaminants. Corals and other sessile marine invertebrates are likewise killed by trace copper, which is why reef keepers monitor for it obsessively.

Where the metal comes from

• Copper or copper-bearing plumbing and fittings, and water that has sat in copper pipes.
• Copper-based fish medications dosed in, or carried over into, an invertebrate tank.
• Brass and galvanised (zinc-coated) metal parts contacting the water.
• Untested rock, substrate or second-hand decor that previously saw copper treatment.

Protecting an invertebrate tank

1. Start with purified water (such as RO/DI) and use a quality water conditioner that detoxifies or chelates metals.
2. Keep all copper and copper medications out of the system entirely.
3. Quarantine and medicate fish in a separate tank, and never move copper-exposed equipment into the invert tank.
4. Avoid brass, galvanised metal and unknown second-hand hardscape that may carry residual copper.