The average copper level in sea water is 0.25 ppb.
The role of copper in sea water is a double edged sword.
Copper is an essential micro nutrient as marine invertebrates requires it for their “blood”.
Due to the tiny amount of copper available in seawater and it is absolutely required by marine invertebrates, marine invertebrates such as coral, worms, by evolution, do not have a “stop switch” for copper intake. That mean they will take in as much as available to them in the water. When they are overdosed with copper, they got damaged and lead to their death, That is how copper medication works.
Now lets look at copper in salt mix. If you take dry salt for measurements, the relative copper level (equivalence to post mixing into sea water), can be as high as 3 to 5 ppb.
That is because after mixing into sea water, most of the copper, some reports shown like 90 to 95% of it, will turn into insoluble copper carbonate, that in turn precipitate out and won’t stay in the water column. That will yield the final copper level to be around 0.3 ppb, which is similar to natural sea water level,
If you get 3 ppb of copper in the tank, it is not a disaster yet, copper can be removed by activated carbon.