Just say a smelter is producing tons of lead, zinc etc, and eventually the fish...

[jasomine]

...in the lake nearby have the? metals in their muscles blah blah

how did the metals GET there?
in the air?
the sewer system?
no idea,
and how does that effect the ecosystem?

 

[Terry S]

Tiny particles of these metals will get into the air and go up the stack during the smelting process. They then land and get washed down into the lake when it rains.

 

[Emily C]

Microbes take trace amounts of those metals into their bodies. Those microbes are eaten by other creatures which in turn are eaten by other creatures further up the food chain and so on. As each creature is eaten, the metals in what they ate are carried into the body and accumulate. Most creatures have a hard time expelling toxic metals from their system