Copper (Cu)
Acutely toxic to shrimp, snails, and many corals. A ceiling parameter — there's no "ideal" copper, only "below the threshold".
What it is
Dissolved copper. Trace amounts (≤ 0.01 mg/L) are essential as a plant micronutrient — copper is built into several plant enzymes — but the line between "essential trace" and "kills invertebrates" is razor thin. Copper is the third most common toxicity emergency in the hobby, after ammonia and chlorine.
Why it matters
Copper enters tanks through three routes: copper-based fish medications (Copper Sulfate, Cupramine — sometimes dosed into the wrong tank by accident), brass plumbing fittings leaching slowly into a top-up reservoir, and certain "anti-algae" treatments that don't advertise their copper content. Once introduced, copper binds to substrate and decor and can leach back into the water column for months — a single dose can render a tank permanently uninhabitable for shrimp.
For shrimp keepers and reef keepers, copper testing is preventive: you check tap water and any new product before it touches the tank, not after.
How to test
JBL Kupfer, Salifert Cu, Hanna copper checker. Limit of detection 0.02–0.05 mg/L. Test tap water before adding shrimp; test any new aquascape rock by soaking it in a small bucket and testing the soak water at 24 hours.
What high and low look like
Below 0.02 mg/L: invertebrates fine, plants well-fed. Above 0.05 mg/L: shrimp dying within hours, snails crawling out of the water. Above 0.1 mg/L: acute toxicity for almost every freshwater invertebrate.
How to fix
The only honest fix for accidental copper contamination in an invertebrate tank is "tear it down and start over". For ongoing low-level leaching from source water, run tap through a copper-specific resin (Polyfilter, Cuprisorb) or switch to RO water.
