I am in the exact same boat. Woke up this morning to ich in my son's 5 gallon, which hasn't had any fish/plants added in 3 months. About a month ago his tank got a new castle, but it spent 5 minutes in boiling water before being added to the tank.
I don't know where it came from.
I haven't had an ich outbreak in 6 years, but last time I did I added 1 tablespoon per gallon of water into a 30 gallon 'community' tank and raised the temp to 85 degrees. In that case I dissolved 27 tablespoons of salt into a quart of boiling water. After it cooled I added 1 cup of the saltwater mixture every 8 hours. I kept this salinity and heat for 4 days after I saw no more spots. That is the key to killing ich, don't stop once the spots are gone!
I have high PH and very hard water here. My fish are accustomed to, and I typically only stock hard water loving fish. Adding salt softens the water, so if you have already soft water, you may need to adjust salt concentrations.
We want our fish to be happy and healthy and live in a perfect environment. Remember though that nature is almost never perfect, and fish are very tough little creatures. They can handle way more variation in water condition than we typically give them credit for, especially in short duration.