When I did my ich treatment I set my temperature at 80F/27C because any higher and they might stress the fish. Goldfish are cold water and, unlike tropicals, can't take the heat as well. Keep the temperature high for 1 month, as well as adding the 0.3% salt solution. At every water change you need to top up your salt. My fish came out all healthy.
Alex may recommend a different length of time for treatment.
The key is to be patient and wait. If you wait enough you will kill all the ich completely in your tank. If you let even one ich parasite live, in 7 days you will have 1,000 to deal with.
The fish you got 3 weeks ago is probably the reason for the ich. It would have been better to isolate him for at least a month, find that he has ich and treat in a quarantine tank.