"How do I fix a dish that has too much salt? I heard that adding potatoes can help remove the salt. Is this true?"
When you want to fix a dish to which you've added too much salt, adding a potato won't do the trick. I love potatoes as much as the next guy, but they are not magical‚ and certainly not capable of selectively sucking the salt out of your soup. For his wonderful book on food science, What Enstein Told His Cook, Robert Wolke actually proved, scientifically that adding a potato did not alter the concentration of salt in the water.
So what's a cook to do? The best solution is not the easiest: build up all the other flavors around the salt. In other words, increase the quantity of your main non-salty ingredients and the concentration or flavor of the salt will diminish. But that's a bit like nearly re-making your dish.
Instead, if it is a soup, try adding salt-free stock or water. Or, if it is a chunky soup, remove about half the broth, leaving the vegetables and meat. Replace the discarded broth with no-salt stock or even water. More vegetables and/or meat will help decrease the concentration of salt as well.
Most cooks try to "cover up" the saltiness by adding acid (lemon, vinegar, zest, tomatoes, etc) or sweet (fruit, carrots, honey, sugar, etc). That's a bit like the fact that perfume was originally invented to cover up body odor! Depending how much too salty the dish is, you may be able to use other strong flavors to bring the perception of the saltiness down (rather like being unable to smell the person beneath all that perfume)‚Äîand you'll likely create a new dish in the process. It's worth a try if you have the time, and are willing to say it's an experiment that may or may not save you from tossing the entire thing.