Zinc, Lead and Other Trace-Metal Toxicity in Aquariums
Beyond copper, metals like zinc, lead, cadmium and aluminium can poison a tank, usually from metal objects or source water. Invertebrates are most sensitive, and soft acidic water makes some worse.
Copper gets most of the attention, but it is not the only metal that can poison an aquarium. Zinc, lead, cadmium, aluminium and others can leach in from metal objects, contaminated decor or source water, and like copper they tend to hit invertebrates hardest. The common thread is that toxicity depends heavily on the metal, the dose and the water chemistry.
How toxic, and to whom
A study of metal toxicity to a freshwater ostracod, a small crustacean comparable in sensitivity to Daphnia, ranked the metals from most to least toxic as cadmium, copper, iron, manganese, lead, zinc, aluminium and nickel. The 96-hour lethal concentrations spanned an enormous range, from about 13 micrograms per litre for cadmium and 25 for copper up to roughly 1,190 for zinc, 3,100 for aluminium and nearly 20,000 for nickel. The free zinc ion itself is described as highly toxic to bacteria, plants, invertebrates and fish.
| Metal | 96-hour LC50 to a freshwater ostracod (µg/L) |
|---|---|
| Cadmium | 13.1 (most toxic) |
| Copper | 25.2 |
| Lead | 526.2 |
| Zinc | 1189.8 |
| Aluminium | 3101.9 |
| Nickel | 19743.7 (least toxic) |
Where these metals come from
- Zinc: galvanised (zinc-coated) metal items, brass, and some metal decor; harmful to fish gills and especially to invertebrates.
- Lead: old plant weights and anchors, solder and some paints; modern plant weights should be lead-free.
- Cadmium and mercury: usually from contamination, rare in the hobby but serious because they bioaccumulate.
- Aluminium: from clay, laterite or untested rock.
- Nickel and chromium: from plated or stainless-steel items immersed long-term.
Prevention and removal
The best defence is to keep metal out: avoid galvanised, brass, plated or unknown-alloy items in or above the tank, use lead-free plant weights, and quarantine or test questionable rock and decor. Use clean source water, favouring reverse-osmosis or deionised water where the tap is suspect. If contamination is suspected, water changes with clean water plus a metal-binding chemical filtration media or a chelating conditioner help pull metals out, and invertebrates should be considered most at risk.