The Differences Between Fresh and Salt Water in Osmotic Regulation in Fishes and Other Aquatic Animals (NEW!)

There is a major difference in the physiology of marine fishes and invertebrates versus their counterparts in fresh water in regard to
"osmo-regulation" = osmotic regulation. Put simply, in freshwater species their internal body fluids are more concentrated than the outside water that surrounds them.  This causes the water outside their bodies to pass by diffusion through their skin and gills into their bodies to dilute the water inside them, which if not counteracted would cause them to swell up.  To neutralize this, the freshwater fishes and other freshwater animals have the ability constantly to excrete the excess water from their bodies through their gills, skin, and urine. 

However, for saltwater fishes and other saltwater animals the opposite is true.  Their bodies are constantly losing water content to the outside water that surrounds them.  They have less concentrated body fluids inside them than is in the salt water outside.  Saltwater animals constantly need to absorb and retain water to keep their body fluids in balance.  Otherwise they would shrivel up and die.  While freshwater animals constantly have to get rid of excess water, the outside water that saltwater animals are absorbing remains in them longer with less water turnover.  So for saltwater animals the uptake of water from their surroundings is much less than it is for freshwater animals. 

This means that there is a likelihood that soluble toxins in the outside water can more readily and quickly invade the bodies of freshwater animal life.  Therefore, what is put in fresh water, such as chemicals and medications that are absorbed through the skin and gills, may be of more immediate potential help or harm to freshwater fishes than it would be in saltwater fishes. 

Conversely, chemicals and medications added to salt water are going to take longer to be absorbed by saltwater animals than those in fresh water.  While what is absorbed by saltwater animals into their bodies is less than for freshwater animals, it may be retained longer in saltwater animals, thereby being a longer-term help or hindrance in this way.

Whatever, in general it appears that chemicals and medications seem to have about the same effect per Standard Dose on the fishes and aquatic invertebrates in fresh water as it does in salt water, despite the major differences in osmotic regulation by these animals in fresh versus salt water.  However, it would appear that for those medications such as antibiotics put in the water for which most of them are active less than an hour, more than what is needed in fresh water will be needed in salt water.  For more information on Standard Doses see Kordon Article "Standard Dose for Liquid Treatments".