Fluval sea products

[425nm]

Copper is needed by living things (its a micro nutrient), just in a very specific quantity. With 3mg/L dosed at 5mL per 200 litres you're looking at:

3mg/L X 0.005L = 0.015mg / 200L = 0.000075mg/L of copper which isn't a whole hell of a lot. However it being a micronutrient I imagine the dose/overdose threshold isn't super high. Which is why you shouldn't exceeding the dosage instructions.

Could you look on the ingredients portion of the label (it appears to be right above the nutrient percentage breakdown you posted) and tell us what form the copper is in?
Some forms of copper are probably more toxic than others.

All this being said. It is a fluval product.