The salt box says 1 tablespoon for every 5 gallons.
I multiplied that by 5 for my tank.
Here is the recommendation which I followed (and it worked):
"How much? I visited websites and read articles on treating Ich in generic freshwater fish, food fish, guppies, loaches, and African rift lake cichlids to name a few. I encountered dosage recommendations ranging from about 1.75 tablespoons to 6 tablespoons salt per 5 gallons of water. One rift lake cichlid importer/breeder uses “1 handful” of salt per 5 gallons of water. I concluded that my fish can probably tolerate more salt than I think, at least on a short-term basis. Based on everything that I’ve read to date, I would feel comfortable adding 2-3 tablespoons salt per 5 gallons if I were also using the high temperature treatment outlined above. If I were using salt alone, I would work my way up to 4-5 tablespoons per 5 gallons. We don’t want to skimp on our treatment if we hope to permanently eliminate this pest. Salt should be added slowly over the course of 24-48 hours or so (always dissolve in a small container of tank water first). Keep a close eye on your fish and perform an immediate water change if they show any additional signs of stress (beyond what the Ich is already causing)."