From A Marine Biologist :
Because water has a natural tendency to move from area of high concentration to low concentration (across a simi permeable membrane = osmosis) Fish face several problems:
Fresh water fish -------- water enters bodies, salts are lost
Fresh water fish:
produce high volumes of dilute urine
Salt absorbing cells (on the gills) actively transport salts into body
Cover themselves with mucus
Marine fishes -------- water leaves bodies
Salts are accumulated (taken in)
Marine Fish:
drink lots of water
excess salts secreted by special cells located on the gills
Produce concentrated urine
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.
Response from the University of Iowa School Of Marine Biology
The only information that I found that stated R/O could be a health risk was on websites and in articles that were advertising a competitive product or an additive that would solve this problem. I did not find any warnings on sites like the EPA drinking water website. This leads me to believe that these companies are preying on peoples lack of knowledge and attempting to scare people into their product. However, I also couldn't find a research article that debunked the commercial sites.
I even found one site (recommended by the EPA) that stated R/O water is
better than soft water for the breeding of some tropical fish because of
the low total dissolved solids.
As adults, we don't get a significant amount of any nutrient from our drinking water. You may want to look at the following websites for more information and a list of water treatment options.
EPA Drinking Water Site Water Quality Association
From Tropical Fish Article: "Note: While soft water is an improvement in that it reduces the clouding and scaling of the glass panels of an aquarium, it does not of itself necessarily provide a suitable environment for the breeding of tropical fish. Authorities indicate that water of low dissolved solids and pH control my be more desirable for breeding, though this depends on the species. Since total dissolved solids content of a softened water is the same as that of the untreated raw water, a supply with a lower dissolved solids content must be gained in some other way. Blending of softened water with reverse osmosis or distilled water may produce the conditions conducive to breeding."
Take care,
Marcy
Marcy M. Seavey
Education Director
Iowa Academy of Science
175 Baker Hall, UNI
Cedar Falls, IA 50614