Hiya, welcome to Permies, don't worry, no bullies here!
Great replies above! I've got a small pond fed by rainwater with roach in them and any kind of wild plant from the surrounding lakes and streams i could find and take home ( only if it was abundant). That way you infect your water with thousands of different bacteria and insects that come along with the plants, creating a very stable micro climate for fish to come in.
I threw in some small fishes and they turned out to be roach, not much of a tasty fish i found, but i didn't have to aerate for four years, and i never feed anything but snails which i lob in from the neighboring veggie patch as a treat. The good thing about roach is that the heron doesn't see them like koi, they're afraid. I have to sit still if i want to see them myself, then they glide in from their hiding place.
Koi are dirty pigs, they will eat all the plants, and go in the mud at the bottom and wuzzing up clouds of muck about, reducing photosynthesis further, eliminating the last remaining plant from creating oxygen, creating a grand environment for toxic algae to bloom in. So you end up with green
pea soup with koi constantly at the surface gasping for air, like manic orange saucages. Many people love this though and happily show me their beautiful pond , wondering why those plants keep disappearing and overfeeding the Koi, because they're "hungry", because why else would they be at the surface trying to eat something...As well an adult Koi shits as much as a small dog.
Roach eat small insects and only start eating plants when water temperatures reach a warmer temperature scientist found out, i forgot what number exactly, but i suspect, they just like fresh newly put out foliage. The big advantage of that is that it leaves the plants intact, more or less. The plants in return filter the fish poop out so algae don't really get a chance. I do have an
underground water storage and an above ground one. If the IBC tote is full, it has an overflow directly into the pond, as well i can open a valve at the bottom and hose it into the same pipe leading to the pond. So when it starts getting hot and dry i always have water in the pond, which is handy, because the veggie patch is nearby. If the IBC is empty i can pump water out of the reservoir underground into the IBC, but that takes electricity. It's a rather stable system so far, even had enough water last year during worst drought in forty years.
Everywhere, i have ever seen ducks, they have a dirty mucky dark green algae ridden pool. I'd hate to have them ruin my system. But as i've said my pond is small. And i have no hands on
experience with ducks. I'd have a race that carefully wipes the mud of their flappy feet before entering the water only eating the labeled plants(haha).
If you're going to do a big pond, it might be different, but when you don't have experience with one thing, taking on two things can become quite tricky. Maybe it would be clever to have a plan B where they could be seperated quite easily if the ducks start mucking things up with their flappy feet full of mud massacring your expensively bought waterplants while they're at it.
The thing is, you've got to carefully plan what to do , because the animals can't help but do what they do, which usually means a lot of damage, and since we trap them, they can't go away somewhere else to do what they do. And they're going to be there all the time doing their "damage".
I think
Sepp Holzer has the best system for ponds i've seen. Did you see his films? Farming with nature its called.
Anyway, whatever you do, good luck and tell us all about it with photos of the build, we'll all try to help commenting and try to be funny(cringe), which isn't very helpful at times, because we like to emphasize our own stories which might not relate to your own situation at all. Everybodies situation is completely unique, weatherwise, climatewise, moneywise, soilwise, etc,etc. So it can be like a maze you have to navigate. But i tell you it's the best thing to do for an eco system to build a beautiful pond and the amount of life that is going to come for it is worth your weight in gold to see.