I
This is absolutely true. Aquatic pathogens are ubiquitous to all bodies of water. However, one should be aware that aquatic fowl in particular may introduce an organism for which the pond's residents have no existing immunity.
How many ponds do you have and what size? Are they lined or earthen-bottom?
The short answer is, too many and too small. I build up, I down size, I build up, I down size, I build up. I just got rid of a dozen ponds and I'm down to fifteen. I've used wood, metal, rubber liners, fiberglass, cast iron, cement, glass, acrylic, if it could hold water, I tried it. Solid shade to full sun. With and with out plants, fish, frogs, ducks, crayfish, snails. With and without pumps.
What body part I would not sell for an earthen bottom pond? Hard to have when you live on a slope that drops ten feet over fifty. Not steep enough for bob sledding, but almost. The one flat place in our yard can't have a pond because we need it for road access. I keep telling my husband, we don't need a truck. He keeps telling me we do if we want to eat.
I salivate over ponds like the one in the photo. I have a friend who has a similar set up, slightly larger with a bio filter, and she has about 20 koi and two ducks. I will admit, her ducks also have a very heavily planted yard to forage in so the plants in the pond are not the only option. While the duck pond the ducks in my yard favor is tiny, my ducks have an acre to forage, so the pressure on the pond is not as great.
I probably should have jumped on the "why are you feeding your fish" band wagon. I never feed the fish. Never.
