Le the tank cycle from the LR
you have. You can add a pinch of flake food just to keep a source of ammonia. Leave the DT without any fish for EIGHT (8) WEEKS. Add a pinch of food every week to keep the bacteria going. Ich cannot live without a host fish. The life cycle is ~30 days but could be longer. Letting the tank cycle for 6-8 weeks with no fish will result in an Ich free DT.
Be sure to QT all new livestock for 30 days to be sure they don't bring Ich into the QT tank. At the first sign, start treating with Hyposalinity