right, this is what i was told and saw with my own eyes, its upto you if you wish to beleive it.
i was told from a breeder that they add a little salt to all of the tanks ecause in the wild there is always a tiny amount of salt in every water, and the fish love a bit of salt every now and again its like a little child on a sugar rush, they seem to love it there colours become brighter aswell. i have always done it with my bog standard comunity tank and the colours are something else.
if anyone does try this make sure it is proper aquarium salt and youy add the tiniest amount.
again this is a personal oppinion, you dont have to beleive this.
p.s if guppies can live in cold water they can survive a little salt.