The recommend high concentration in reef water for copper is only 30 parts per
billion. That's not much. The hobby grade test kits can't detect copper much below 100 ppb. For some coral and reef occupants, copper causes problems as low as 10 ppb (& some research is finding levels lower than this can be a problem for certain coral). The form that copper takes in the reef tank makes a big difference. When copper is tied with dissolved orgnanics it is much less toxic than when in the ionic form which is what you find in salt mixes and many chemicals used to add copper for treatments of parasites....etc.
