The Truth About RO/DI Water

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Pure Water – fact vs. fiction

Written by SleeplessLwd for Aquarium Advice

I want to clarify something that has come up several time in previous post that has been bothering me since the first time I read it. The answers I was getting when I asked them to explain their theory did not make sense. So I did a lot of research which involved talking to a lot of different people and e-mailing several people and this is what I found out.

Several people are telling people that R/O or Pure water will pull nutrients and minerals from the gills of the fish. I spoke with people from labs, water companies, even a University for Marine Biology and even Marine Biologists, and I have received several e-mails back from different places that I requested information on this matter from, and the bottom line is: R/O or Pure water does not draw the minerals and nutrients from fish. Water can dilute and dissolve, but it does not absorb the way the people in these posts are saying it does. The only problem I’m being told that R/O or pure water seems to create with fish is that it doesn’t have enough nutrients and minerals for the fish to survive. Listed below are some of my replies from my requests for information to back up what I am saying.

We do not have information about fish, but I can tell you that RO water or pure water is not harmful to the human body. It does not leach minerals nor does it make the blood acidic. As soon as pure, RO, or distilled water is ingested, homeostasis in the body immediately and effectively adjusts all body fluids to the correct salt content. You will need to check with a fish expert to see if fish have the same homeostatic ability.

Kelley Thompson
Public Affairs Assistant
Water Quality Association

Dear Elaine,

Thank you for your inquiry. When RO water is produced, has almost zero elements and minerals in it besides the H2O. If you put this water in with fish without adding trace elements back in will harm the fish because they don’t have the required nutrients from the water. It won’t strip the elements from the fish though.

If you are using RO water for saltwater, most aquarium salt will have many of the trace elements needed by most marine life, of course you would have to add additional trace elements as needed.

If you are using it for freshwater, you will need to add freshwater trace elements or water stabilizers like RO Right, to get some of these minerals back into the water. If we can be of additional assistance, please contact our Live Aquaria Department at 1-800-334-3699 or via email. If you should need assistance while on our web site please feel free to contact our Live Help. Our Live Help is available Monday through Friday 11 A.M. 2 P.M. CST.


Drs. Foster & Smith
Aquatic Services

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 M. Seavey
Education Director

Iowa Academy of Science
175 Baker Hall, UNI
Cedar Falls, IA 50614

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