Question

What happen when a star fish is put in a lake

Answer 1

Because of osmosis it would die, not just in a lake but any fresh water.
If you put a freshwater fish in salt water the salt would draw the mouisture out of its body making it die.
Same thing happens if you put an ocean water fish in freshwater except the salt within its body would draw to much fresh water in, and its body salt would fall out into the freshwater and later die.

This all has to do with PH and the PH balance so you can read more about that to understand.

Answer 2

They might die because they need salt water to survive.