Why copper needs completely different thresholds by stocking
Copper is one of the few readings where safe limits depend enormously on what you keep. Fish generally tolerate low copper concentrations without obvious issues — shrimp, snails, and other invertebrates, by contrast, are extremely sensitive even to tiny fractions of that amount, since copper is directly toxic to their metabolism (including hemocyanin, their oxygen-carrying protein).
For a tank with shrimp or snails, the practical rule is: the target is 0, exactly like ammonia or nitrite.
Where copper comes from
Copper pipes in older buildings: The most commonly overlooked source — tap water from old copper plumbing can carry meaningful amounts of copper, especially if the water sat in the pipes a while.
Copper-based medications: Many classic ich treatments and algaecides use copper sulfate as an active ingredient — an absolute no-go for shrimp tanks.
Contaminated equipment: Used gear that previously touched copper-based medication can leave residue.
Some fertilizers: A few trace-element fertilizers contain small amounts of copper as a micronutrient.
What to do about elevated readings
Test tap water in advance: Especially in older buildings, run a copper test once before your first water change for a new shrimp tank.
Let the tap run briefly: Before drawing tap water, let it run for a moment to flush out water that's been sitting in the pipes.
Use activated carbon: Effectively binds dissolved copper and makes a good emergency measure for known contamination.
Water change: Dilutes the concentration, but doesn't fix the source permanently — track down the cause in parallel.