Set up a 10-20 gallon bare-bottom tank with a cycled sponge filter, heater, and PVC pipe hides. Match parameters to your display. Drip-acclimate the new fish, then begin prophylactic treatment (copper for marine, salt+temperature for freshwater) for 4-6 weeks before moving to display.
See our complete Quarantine Protocol guide for full setup, treatment regimens, and the marine vs freshwater differences.