Question

what osmotic regulatory challenges would a fish living in freshwater have versus a fish living in salt water?

Answer 1

FIshes are found in both fresh as well as marine waters. The fresh water has much less salt content than the marine water. Thus the fishes living in these two waters have different mechanisms to overcome the osmotic pressure.
A fish living freshwater would have more salt inside the body as compared to its surroundings, this condition is called Hyperosmotic. Therefore, the main challenge for freshwater fish is to retain enough salt from fresh water. The salts are necessary for normal bodily functions, they excrete lot of water into the surrounding waters.
A marine fish on the other hand, has the opposite problem. It is hypoosmotic, meaning has less salts in the body as compared to the surrounding water. Therefore, the main challenge for marine fish is to expel enough salt from saltwater. These fishes have get rid of salts which come in from the surrounding water.