One of the things to consider with a pond, are bird feces (or even other animals ) being carriers of parasites.
Anything that can eat a fish, may carry the parasite in its gut, and drop it into a pond, miles away from where the phage has been picked up.
a virus, a protozoan, whatever
and....
Because all my fish are locally wild caught, and usually brought to me within hours of capture at least 50% arrived with some sort of blood sucking thing attached, that in nature is only a minor irritant, but in the context of a closed system, like a tank, or pond becomes much worse as it multiplies, and constantly reinfects the same fish over and over becoming lethal.
Lernae (anchor worm) above.
Some burying protozoal parasite like ick above.
Both were prevented in spreading by bringing the salinity to at least 3 ppt
Raising he salinity was done by adding 3 lbs of salt per 100 gallons.
Although larval Lernea were compromised by salt, the adult Lernea laughed it off, (unaffected by salinity) so needed to be physically removed.