Well first, freshwater dips is not a method to rid a fish of Ich. At best it may provide some temporary relief. Second, it is unlikely placing a fish in a sump will stop the spread unless you isolate it from the DT. But at this point, you will probably be stressing the fish out more, which is not what you want to do. If you want to get rid of Ich, you have a couple of options. Treat just the PBT and hope none of your other fish are infected, or pull all of them from the DT and treat them all in a HT. You would need to leave your tank fallow for 10-12 weeks. Treatment methods include copper (cupramine), hyposalinity, chloroquine sulfate, and tank transfer method. Which way you go is up to you and what you are comfortable with.