Whenever you bring newly-purchased (or newly-obtained) fish for your pond(s), follow the procedure of allowing them to “acclimatize” to your pond’s temperature BUT do not introduce them to the pond where you keep your “old” (or “current”) “stock” of fishes. Place them in a “quarantine” or “isolation” pond instead, and keep them there for a few weeks, so that any “hidden” diseases will manifest themselves.
Make sure that your quarantine equipment (i.E., pumps, filters, and even the water itself) are in perfect order, so that the newly-purchased (or newly-obtained) fish won’t be unnecessarily “shocked” or “stressed”.
After a few weeks, when you are quite confident that the fish have no “hidden” diseases, you may introduce them to the rest of your “current” stock.
It is also a wise idea to maintain two or more quarantine or isolation ponds so that if you purchased (or brought) fish from two different “sources”, you may allocate one quarantine pond each for the two sources. That way, there won’t be any “cross-contamination” should a hidden disease manifests itself, and it would easy to pinpoint which “source” supplied the ailing fish(es).