Take out all the fish and put them in a smaller tank, which wont be so expensive to treat. treat all the fish, and raise the temperature of the 300 gal tank to help the parasite along in its life cycle.
Raise the temperature of the hospital tank too while your treating the fish, and it will hurry it along. the life cycle of the parasite goes by faster if its in warmer water.
treat the fish until 3 days after the parasite is gone.
let the 300 gallon tank water filter and sit for a few weeks to let that parasite die.
after there is no sign of itch on your fish and you have let the 300 gallon tank sit, you can slowly start introducing your fish again.