Nature already does this. Salmon. They spend their lives in both environments. Edit: I guess people need to go fishing more. Salmon do spend time in an ocean, then spawn in fresh water. Thanks for the negative rates for my actually giving an answer as well as an example as compared to the other answer posted here, which does neither. Edit: It's possible to have the fish switch. Here's more than a few references: http://books.google.ca/books?id=2MYCAAAAIAAJ&dq=scientists+turn+salt+water+fish+into+fresh+water&ots=FUVX8QS5sj&sig=4i7F9zKpX13hcSkq-5HVzPSk3fU&prev=http://www.google.ca/search%3Fhl%3Den%26q%3Dscientists%2Bturn%2Bsalt%2Bwater%2Bfish%2Binto%2Bfresh%2Bwater%26btnG%3DSearch&sa=X&oi=print&ct=result&cd=2&q=scientists%20turn%20salt%20water%20fish%20into%20fresh%20water&pgis=1 http://www.madsci.org/posts/archives/jan99/916188046.Gb.r.html http://www.madsci.org/posts/archives/jan99/916188046.Gb.r.html http://news.ufl.edu/2000/02/10/stingray/ The process is even patented here: http://www.freepatentsonline.com/6016770.html It's more difficult going from fresh to salt, but HAS been done.End of argument. The question asks if it's possible. It is.The question doesn't specify any types of fish, the sources don't. You lose. Get over it BB.

Assumption: The question does not mention a specific species, therefore the question is asking about freshwater fish, in general. No. You cannot turn any random fresh water fish into a salt water fish. You cannot turn a salt water fish into a fresh water fish. Each species of fish has a specific range of environmental variables that it can survive within. While a fish may be used to one set of extremes of those variables and you can accustom said fish to the other end of the extremes of said variables by adjusting its environment over time, you cannot acclimatize a fish to living in an environment whose level of saltiness -- and other chemicals, temperature, variables, etc... -- is outside of the range for which that particular species can survive. Fresh water fish and salt water fish have evolved to survive in the salinity -- the level of saltiness -- of their native environments. Salt quantity isn't the only difference between salt and fresh water. There are many other variables, ranging from other chemicals to various organisms. These would have to be adjusted, too. There are a handful of species that can go between fresh and salt water, but most of those require being in one or the other (or both) for some period of time throughout their life cycles. Some references (from scubabob, whom I owe a "thanks" for making me look into this some more): http://www.freepatentsonline.com/6016770.html This patent covers a process for acclimating saltwater fish to fresh or brackish water. It explicitly calls out that not all species are compatible with such a change in their environment. The subject of fresh to saltwater conversion or acclimatization is completely avoided. (Coincidentally, the patent has expired due to non-payment). http://www.madsci.org/posts/archives/jan99/916188046.Gb.r.html This article posits that it should be possible to acclimatize a saltwater fish to freshwater, but concludes that "... but it might be difficult to acclimate fresh water fish to salt water. The fresh water fish may just not have the biological equipment to excrete the excess salt it would ingest and might therefore dehydrate." These articles and further research shows no evidence that anything but very specific species are capable of moving from freshwater to saltwater. Of those species, I could find none that move from freshwater to saltwater permanently and, certainly, no evidence of a acclimatization regimen that might enable a random freshwater fish to live permanently in a saltwater environment.

After reading all the arguing I'm not sure I should put any response on here...but, the only tropical fish tank species I know of that can be changed to saltwater is a mollie. Whether acclimating or not, I use mollies to establish and stabilize my saltwater tanks. Straight from fresh to salt tank, they do very well. I set up tanks as a side job and have had very good luck doing this. If later you wanted to add more to an established salt tank, be it mollies or saltwater fish, you can. If you are worried about getting rid of the mollies later try a fish that eats other fish. If an anemone or lionfish can fit a mollie in its mouth it will eat it. I'm not saying that I am an expert. What I am saying is that this process is possible, and have stated the species of fish it works with. If this does not help answer your question, I'm sorry. If it does though I'm glad I could help.

I have successfully acclimated a green (Leopard?) Puffer and a Figure Eight Puffer from my freshwater tank to my saltwater tank and have been very happy. The process was over a 7 day period using the drip method of gradually using water from my saltwater tank each day. I started with 5 gallons of water from my fw tank and changed out approximately one gallon per day until the specific gravity from the adjustment tank and the sw tank were equal. The puffers continued to eat, though at a lesser degree, through the entire process. The Puffers were introduced to the sw tank and have lived there for 6 months with no difficulties. My sw tank has live rock and anemonies (sp?) that the puffers have left alone which is quite surprising. Their colors have become much more alive and the fish seem quite happy and readily eat the same live food as the other sw fish. I have searched for a method of doing the same process with Clown Loaches but haven't found any so I suspect that isn't possible but may just subject a couple to the test. From what I've read, they live in similar water conditions that the puffers were reported to live in but of course, I may be incorrect. Any ideas would be appreciated.

I have always heard that it was possible, but with just a few species. The puffer is already a brackish fish and the mollies are from the Cape Fear river, which will run backwards with a good wind. So I am not surprized. I was thinking of trying it with the Chinese Hi-Top "shark"/carp. JEH

some. Some people use small amounts of salt in their tanks as a sort of preventative antiseptic...Mollies and some other brakish species are okay with it...but I'd be careful...also if you ever add water to you aquarium afterward make certain that you add distilled water so it doesn't just get saltier and saltier.

Well, I for about a month now have also been acclimating a freshwater leopard puffer that my mom wanted to get rid out of her tank cuz it killed her crabs, so instead i tried to play chemist, and it worked, he's hit 1.022 today and ate a huge piece of krill. i acclimate him with water from my running 200 gallon salt tank so he has had the increasing amounts of chemicals from the start. Just a rio 600 and a heater. He eats marine algae, krill, brine, u name it. The cool thing about this is i am 100 percent positive my fish guy buys these puffers as tank raised, so its not like he grew up in brackish water, he spawned in freshwater. I am in a way proud of this accomplishment and will be introducing him into my tank this week, and happy to see that one other person has done the same thing. And for any of you people griping at each other about who has the better answer, get a fucking life. They are fucking fish! I have alot of money invested in my tank and my fish and you people make it so dramatic. Respect peoples ideas, help other peope learn about the hobby......thats it its a HOOOOOBBY, not something to fight about amongst people your never even gonna meet. So, if you want an answer from a guy who could give a fuck about what other people say...yes buddy, it can be done...pick your fish wisely, take your time and do something other people will think you are neat for.

People do this for fun? lol