You will need to place fish in Quarantine tank and treat with Coppersafe or Copper Power at therapeutic level 2.25-2.5 For a FULL 30 days (do not interrupt this 30 day period) monitored by a reliable Copper Test kit such as Hanna Brand- No API brand. Also monitor Ammonia levels while in quarantine with a reliable test kit and add aeration during treatment using an air stone.
The display tank will have to be kept fishless (FALLOW) for 6-8 weeks to assure the existing parasites go through their life cycle without a host fish and die off
A quarantine tank can be as simple as a starter kit from Walmart which most of the needed essentials.