When two solutions are separated by a semipermeable membrane the weaker solution will always dilute the stronger solution. The greater the difference between the two solutions the greater the osmotic pressure as shown in the diagram above.

So how does this affect fish.

In freshwater fish, especially the ones from softwater regions they need to be able to regulate this process or water will invade their bodies. To understand how this happens, look at the diagram and imagine that the weaker solution is the soft water, the semipermeable membrane is the fishes skin and the stronger solution is the fishes own bodily fluids.

The surrounding water is constantly trying to invade the fish and dilute its fluids because they contain various salts and minerals. To compensate for this, fish that live in soft fresh water have evolved bodies that quickly get rid of excess water and at the same time hang on to the important salts.

They do this by passing lots of very dilute urine almost constantly. Their kidneys and other organs have evolved to do this over thousands of years. And in their own environment the system works perfectly.

Now just think what happens if such a fish was placed in hard water, or worse still water with some salt added to it. All of a sudden the surrounding water is the stronger solution and the fish the weaker solution. So the fluid within the fish leaves, helped by the fishes own body because of how it has evolved and the fish finds itself having to try to get rid of excess salts but again its body has evolved to hang on to salts not get rid of them. This actually causes the fish to dehydrate.

Obviously this puts the fish under a great deal of stress and eventually it causes some damage, most commonly kidney stones, and ultimately kidney failure once this happens a premature death soon follows. This is what I mean when I say that most freshwater fish do not have the physiology to deal with salt being added to their water.

For fish that live in a salty environment (marine and brackish water) the situation is slightly different. they have evolved in a different way.

Their bodies have evolved to get rid of excess salt and hang on to the water, the main ways they do this is by drinking lots and passing very little but very concentrated urine. Again in their own environment the system works perfectly. But if a brackish water fish is placed in fresh water it still takes in lots of water and its kidneys still produce very little but concentrated urine. The result again is stress on the fish.

Ultimately the osmo-regulation system breaks down and the fish takes in more fluid than it can get rid of (Dropsy). this is one reason why so many Mollies fall victim to this condition when kept in a salt free community tank.

This is why it is very important to keep fish in the proper conditions and why it is not a good idea to routinely add salt to freshwater aquariums.