[Dentoid]

Basically osmoregulation is keeping water and dissolved solutes in balance across a semipermeable membrane. Water wants to move across a semipermeable membrane to the side with the highest dissolved solutes, ie the saltwater side. Fish cell walls are like a semipermeable membrane but they contain biological pumps that maintain a balance between water in their cells and external dissolved solutes. This is called active regulation. Since fish live in a hypertonic solution (saltwater) their cells and kidneys have to expend energy to pump salt out of their cells so that they don't dehydrate.

These "biological pumps" if you will require energy to do their masterwork and by lowering the salinity of the fishes environment their cells don't have to work as hard. In a healthy fish, osmoregulation is a piece of cake and marine fish have evolved to handle the stresses of dissolved solutes in their environment. However, a sick fish is taxed for energy as it tries to fight off disease. In a case of active regulation-vs-disease-vs-immunity, active regulation is going to win while the fish dwindles to the disease.