Could you go to PetCo, buy some elodea, and place it in a salt water fish tank and have it live successfully? Why or why not?

No

Elodea plant is a fresh water plant that cannot tolerate salt water and will die when placed in salt water. When elodea plant is placed into salt water, it is like placing it into a hypertonic solution. The water inside the elodea cells will move out into the surrounding water, and the cell membrane will be removed from the cell wall. Also, the amount of water that the elodea plant will lose will be more than the water that is entering it and the plant will be unable to survive with the salt in its cells.