Adding arbitrary amounts of salt to a tank is not good, it is a myth that it prevents disease, seldom do most beginners or even some experienced aquarists have the means to check the levels, fluctuating salt levels cannot be good to fish.
Also, adding salt to a tank raises the conductivity which creates havoc on a fish's osmotic process.
People are lulled into a belief that salt is good for their fish because the fish do not get sick, and they are adding salt. SO if they are adding salt and the fish are not getting sick then it must be good right? Not true.
I used this example at my store. If I have a pair of fish about to spawn, and I add an ounce of some household chemical, let's say,,,ummm, listerine mouthwash, and it doesn't hurt the fish and then they spawn, then couldn't I say that adding listerine induced spawning?
Or I could add an ounce everytime I do a water change to a 55 gallon tank of fish. And the fish continue to do good regardless. Which they obviously would have done without the listerine. So,,,couldn't I boast that I add listerine to my tank to ensure my fishes' health?
The point of this absurd analogy is that just because you add something to your tank and the fish do not get sick does not mean it is good.
Adding salt causes the conductivity of the water to rise, it plays havoc on the osmotic process of the fish, and the fish has to work harder to survive. But because in most cases the fish does not fall ill, or ich goes away, people think it is good for them.
As for guppys, what few people realize is that guppys available in most retail outlets come from the far east in singapore and hong kong. They are bred and raised in nearly full strenghth salt water. Then as they make their way from the far east to your retailer they are moved from salt to fresh, and in some instances back to salt and then to your store in fresh. This is why they do not do well in most cases by the time you get them home. The life expectancy for imported guppies is about a week to ten days after purchase. Adding them to heavily salted water when you get them home sometimes staves off the inevitable.
Salt may appear to be good for fish, but in the end it does not benefit them, it is unnescessary, and if you are not careful about measuring it then it will build up in your tank and problems will ensue. Just look through aquarium forums all over the internet or go to a store and listen to all of the people having problems keeping fish alive. And how many of them are adding salt among a plethora of other snake oils?