Ignore my ignorance here please
 

What is the reason a saltwater fish will not survive in FW.????
Something to do with the gills?
My buddy just asked me after seeing my tank and i cant give a good answer.

start here http://en.wikipedia.org/wiki/Osmoregulation

OMG! That's so interesting!

And, some fish have the ability to move between marine and freshwater environments. One very common example is Salmon. Another is the common Molly, which can live quite comfortably in either, and switch back and forth.

Although those that switch from fresh to salt or vice versa have to undergo some pretty intense internal physiological changes and must be acclimated. You cant just dump a molly in saltwater and expect it to live long.

They Can...Salmon :)
Seriously though
Salt...
Osmosis is a term used to describe the physical property exhibited when a difference in concentration exists on either side of a semipermeable membrane. This is stuff we learned in high school but forgot. Water will flow through the membrane in the direction that will try to equalize the differences in concentration. So, water will pass through the membrane, moving from the side of low concentration to the side with the higher concentration. This is the same mechanism that is working in your fish question. The salt water fish will die because too much water is entering it's body. The fresh water fish will loose water when placed in salt water.


NOTE TO SELF..READ WHOLE THREAD TO SEE IF PEOPLE ALREADY GAVE YOUR SAME ANSWERS :)