Losing fish is never a pleasant experience.
Not knowing exactly what caused the fish kill, the logical course of action would be to disinfect the entire pond.
Completely drain and thoroughly clean the pond. Disinfect the pond, including all of the components- skimmer, pump, biofalls filter, entire interior surface and plumbing (as best you can) with a solution of common (5.25%) household bleach and water mixed at a rate of 1.00 - 1.25 oz bleach to 1 gallon of water. You will probably want to replace the filter media (lava rock, bioballs, etc.)
Allow pond and components to completely dry for a week. At this time, it should be OK to refill and let it begin to cycle before adding fish. You can jump start the cycling by adding a cup of common household ammonia to the pond water. This will speed up the colonization of the beneficial bacteria.
This should eliminate any pathogen that may be present in the pond at present, including virus.
It appears that a disease was spread when you mixed fish from different sources.
"Everybody is ignorant, only on different subjects." - Will Rogers