Just one example, but if copper levels get too high, you essentially get heavy metal poisoning in plants and livestock.
Invertebrates are less tolerant to heavy metals, even though they are required for life.
As the saying goes, the dose makes the poison.
How much is that amount? I'll provide some data: I've yet to have seen a single verified case with the standard trace mixes for shrimps(Amano, Fires, CRS high grades etc), which covers most of the commonly kept species and fish are even more tolerant. Shrimp and fish die for MANY reasons in non planted tanks that are not dosed ferts at all.
I've gone pretty high within reasonably errors most might make for K+ (100ppm), PO4(10ppm), NO3 160ppm, Fe 6 ppm(used Plantex CMS+B and SeaChem's) this yields about 0.08 ppm Copper. But, few need to dose 10 X even a super rich trace dose. So typical ranges might be 0.008ppm or less.
So a few parts per Billion.
Not enough to cause any issue for shrimps. Certainly not for fish or plants.
This is some hard data generated over about 15 or so years since around 1996 on many aquariums, and by various folks who make common dosing errors and mistakes. Fish and shrimp were fine.
Wet's dosing calculator:
And dosing and accumulation vs time and water changes: