Should I add salt to my guppy tank

Add 1 tablespoon (Tbsp) of salt per 3 gallons of water. You can pour the salt directly into the aquarium or hospital tank, but some people like to dissolve the salt in a small cup of water first. This level of salt is like using Neosporin topical ointment for a small cut (in other words, it’s not very strong). (source)
Yes, salt reduces the toxicity of nitrite to freshwater fish. Salt is a compound containing sodium and chloride (NaCl). Studies have shown that chloride reduces the toxicity of nitrite to fish, so salt can be added to provide the chloride ions that offer the fish some protection against the nitrite. (source)
Although plain aquarium/tonic salt (sodium chloride) is sometimes suggested as a good way to increase hardness and improve buffering, it in fact provides very little of either. Marine salt mix, on the other hand, will raise the pH and carbonate hardness quite significantly. (source)
As a general rule, salt should not be added to a freshwater aquarium. Salt is an effective medication provided the fish you are treating are salt tolerant. Salt should never be used in aquariums with live plants. Alternative medications should be used with fish that are not salt tolerant. (source)
A teaspoon per 10 gallons of water will support the healing of scales by helping the fish maintain the proper level of salt in its blood. In addition to this use, salt may be added to treat common ailments of freshwater aquarium fish, including ecoparasites, ich, and Costia. (source)
Aquarium salt is a general term that covers a range of salts you can add to the water in your freshwater aquarium. Standard sodium chloride, known as NaCl, is just one of them. (source)