All marine creatures require freshwater just as we do to survive, they just process it differently. Since their bodies are less salty than the water surrounding them, to prevent the loss of needed freshwater they take in seawater, process it to eliminate the salt molecules, and then retain the freshwater to maintain a balance with nature. When the salinity of seawater is lowered, or hyposalinity is applied, it results in a lowering of the osmotic pressure of the water at the same time, thus the related name Osmotic Shock Therapy (OST). Fish and a few other sea creatures can withstand and adjust to this change in pressure, but protozoan (Cryptocaryon/White Spot Disease, and Brooklynella/Clownfish Disease), dinoflagellate (Oodinium/Velvet or Coral Fish Disease), and flat worm (Black Spot Disease) ich organisms cannot. Reduce this necessary pressure, particularly rapidly, and they literally explode! Although delicate corals and invertebrates may not immediately rupture as ich parasites do, these too are marine animals that cannot tolerate exposure to low osmotic pressure, resulting in a rather quick death.
