My attempt at explaining Osmosis and how it effects your fish.

OK to try and put it in more simple terms, water is the solvent molecules and the stuff dissolves in the water (salts etc) are the solute particles. What happens is if you have a high concentration solution on one side of a membrane and a dilute one on the other, the solutions tend to want to equalize, so what happens is that the solvent (water in our case) moves from the dilute side to the concentrated side thus diluting it (and concentrating the side it left) until both are equal. This is what the osmotic pressure is.

All right, so with fish, you got lots of membranes. For example you have gills and skin and fins etc. etc. Now with freshwater fish, the insides are more concentrated with stuff then the water around them. Their bodies have salts and stuff in the form of blood, the foods they eat, and whatnot. So freshwater fish are constantly taking in water into their bodies due to osmosis (dilute water comes in to try and dilute their bodies). Therefore freshwater fish constantly have to be "shedding" water from their bodies in order to survive. This happens in several ways including urine and breathing to name a couple.

Now when you add salt to your tank water, it raises the concentration of salts in the waters around the fish and so the differences between the concentration of the water and the inside of the fish are reduced some and therefore the osmotic pressure is lowered. Less water is taken in so the fish doesn't have to work as hard to rid their body of this excess water.

This is one reason why adding salt to a freshwater tank can be helpful. Your sick fish is already stressed and weakened, so by lowering the osmotic pressure by adding salt, you help your sick fish by giving its body a little easier time in dealing with that problem and more energy towards getting better or healing itself.

Marine fish have the exact opposite problem. The ocean has a much higher concentration of salt in it than in the fish's body, so the osmotic pressure is constantly trying to draw water out of the marine fish. Therefore, to keep from dehydrating, a marine fish is always drinking water to replace lost water. Marine fish have ways of getting rid of the excess salt. So next time someone asks you "Do fish drink?" now you know the answer and it depends on if you are talking about freshwater or marine fish. ;)

From jude@newton.pconline.com
Date: Sat, 7 Feb 1998 02:59:01 -0600
From: Nathanael Henderson
To: [ptimlin@yahoo.com] (patrick timlin)
Subject: Re: What is the result of using distilled water?
Newsgroups: rec.aquaria.freshwater.misc

Hi, Patrick.

Freshwater fish also tend to use their gill filaments as ion exchange sites, taking on salt to replace what's lost in urination, etc. Since disease often comes with a boost in overall metabolism (and presumably demand on the urinary system) part (or even most) of the benefit of adding salt for stressed fish may be from providing an increased supply of salt in the water instead of the modest change in osmotic pressure. (For instance, a 1% addition of salt would have a fairly modest affect on osmotic pressure, but has been found to cut shipping losses by as much as 90% in commercial fisheries.)

As a recent experiment I put a paradise gouramie in a bowl of deionized water for about a week--he pulled through without a problem. I'm very curious about the coping mechanisms of blackwater fish--many come from almost distilled water in terms of dissolved minerals. The only research I've been able to find is for the much more economically 'interesting' food species like trout.

Nathan H.