Perhaps the hobbyists most important technique
Quarantine is the best method to reduce disease introductions. Let me outline a quarantine facility and protocol as if the fish we were discussing were quite valuable.
First, a large holding tank is required for Koi, 100-150 gallons is good.
It should have a fully cycled filter sponge or other type of effective nitrification system, fully operational. A quarantine with bad water is infinitely worse than no quarantine at all. Consider the article on Bioseeding.
Temperature should be 72 - 78 degrees in quarantine. pH should be supported with Buffitup buffer dosed according to label instructions. You should dose the quarantine with 0.3% salt (Noniodized salt at 3 teaspoons per gallon).
The quarantine should be completely covered because Koi like to jump out, and that can be costly.
You should check Ammonia and pH in quarantine each day. Ammonia should be ZERO.
You should feed an antibiotic food during the stay in quarantine.
The tank can be safely treated with Dimilin while in quarantine, even with the salt, etc. This will stop the introduction of Lerneiid and Branchiurian parasites.
Finally, the quarantine should be of a 14-21 day duration, so long as water quality can be supported with changes, etc. as needed.