The level of copper in Flourish Comprehensive is minimal. This is important for fish too, as fish cannot tolerate levels of copper that are safe for humans in drinking water. Provided you do not overdose, you should have no problems from copper, or any of the other heavy metals that are in plant additives (iron, manganese, zinc, etc).
It is best to use less than the recommended dose, aside from the above. We tend to use plant additives assuming we have to, but when one lessens the amount, or even stops in some cases, it is amazing how well the plants may still grow without it. Not suggesting you not use it (I use Flourish Comprehensive), just under rather than over dose.