what kind of fish filter/convert saltwater into freshwater?

There are really no fish that go from saltwater from freshwater. Though, some people do convert Mollies from Freshwater (Brackish) to completely saltwater.

Um, none. How would they do that, drink saltwater and pee freshwater? XD

There are fish that can live in both saltwater and freshwater, if perhaps that's what you mean? Also fish that like brackish water (which is saltier than freshwater but not as much as saltwater).

Fish don't do that. Their bodies would have to somehow extract salt from the water as it passes over the gills, and taking in that much salt would kill any organism.

all salt water fish do that in their own bodies, they extract and excrete the salt as part of their biology .
freshwater fish do something that's sort of the opposite and extract and retain some of the salts in freshwater.
note: freshwater isn't really fresh usually, and has some salts in it.

EDIT: look up osmoregulation in Wikipedia , for more info.
and all of you , stay in school, pay particular attention in science classes, if in college, take at least one extra science class even if it's not in your major requirements.

none?

what?

none because saltwater fish will not tolerate too much salt and freshwater fish can not filter it into freshwater to breath.a archer fish or a figure 8 puffer and some brackish fish can do that