At first, what we are saying could turn out to be quite dangerous for the fish, especially since both types are not used to different environments. That is, freshwater fish would not survive in the salt, and vice versa. In fact, there is more than one case in which animals have died from making this change. Acts that, in a certain way, can be paid for.
In any case, it would be convenient to inform yourself well, since solutions for everything are recently being invented. Above all, for our fish. Without going any further, a few years ago a product was presented thanks to which we could mix fish fresh and salt water. The key was that it managed to "adapt" the water so that it did not affect the species. A very successful achievement.
Since then, a lot of progress has been made. You can check online for ways to mix the two types. de peces. The information is not abundant, but the use of the magic water, which is the fact of adding electrolyte to fresh water in order to change it and not affect health. Something that is still under investigation (it is difficult to reach these advances) but that, if all goes well, could be put on sale sooner than it seems.
In any case, we recommend that you search the net. There is products in experimental state with which you can try, although we do not guarantee that the animals will survive. Power, you can, but the results are not totally reliable. We will have to wait a little longer.