before i do anything else, osmosis is roughly the process by which water moves through a permeable membrane from areas of higher to lower impurity. osmotic pressure is driven by the difference in purity levels on each side of the membrane.
Dont be so quick to rule it out guys, i have said a million times that certain morays can go from FW to SW and then back to SW again. I did that with mine since i was forced to, and hes happily eating. Also, certain bumblebee gobies can go in either, although it is likely these may be classified as a seperate species one day (like a FW one and a SW one) .
i don't think it counts as a sucessful acclimation to FW if you had to switch him back before he starved to death. additionally, there's quite a difference between fresh and brackish waters. for starters, the second has salt in it.
there are different gradients of brackish, but if its got any salt, its not fresh. i've not heard of any moray that can be happily acclimated to live in full freshwater its entire life, but there are 2-3 species that can do quite well in various levels of brackish.
there are in fact 3-4 species of bumblebee gobies and some of them are in fact classified as 100% brackish while others in theory can be acclimated to freshwater. the trick is identifying which species you have (a process complicated by the potential presence of hybrids in any batch of captive-breds)
However, evolution takes millions of years, especially to place a FW into SW. I have seen many brackish fish go into full SW and FW before as well, but this is very different form a full switch. The way that fishes osmosis works basically rules it out. A freshwater fish would be sucked dry in saltwater, and a marine would likely inflate to its doom.
Think about this: freshwater rays are closly related to saltwater sharks, however fresh rays cannot tolerate any salt. They evolved to tolerate freshwater, and even though they were once in saltwater eons ago, their osmosis has changed them, and it cannot be changed back. If it becomes favorable in the future, then they may begin a slow change back to saltwater, but itll take millions of generations, not just one or two.
good points here, however. i'd just like to point out that it isn't that the osmosis is different for each case, but different species have evolved different biological mechanisms to handle osmotic flow of water. some examples:
--many freshwater fish have non-permeable scales that keep too much water from entering through their skin. very few saltwater fish have scales and if placed in freshwater, the outside water would quickly begin to push its way through their skins.
--fish that have evolved to live in salty water must purify any water taken in with their kidneys before their bodies can use it. this creates an osmostic pressure that pulls water out of their bodies and to counteract that, salty water fish constantly swallow water as they swim. a freshwater fish put in salty water would also experience this removal of water, however swallowing water wouldn't help them in the long term because the freshwater fish's kidneys aren't evolved to handle that sort of heavy load.
