I'm new to both hobbies, but I've had really good luck with ducks and goldfish. I honestly got the goldfish as feeders for the emaciated ducks, but then I started to really like the fish and wanted to help them survive, so I've been working on this.
I have 3 full grown ducks that I rescued as neglected adults. They came to me starving and now have full gloss feathers and walk without their knees buckling if they trip on something. They are apparently about 10 years old. My goldfish are only about 4 months old and there are about a dozen between 2 and 4 inches (the smaller ones spent some time in a smaller tank and didn't grow as much), and I have minnows that have already reproduced.
Because this is a budget project, the hardest part has been keeping up with leaks, but I think I have them all figured out and am looking at how to "dress it up" so that the cinder blocks, tub and bucket are hidden.
I am using a 100 gallon pond liner, with a 30 gallon connector pool, a "waterfall" box feature, four 12"x16"x4" ldpe tubs full of lava rock, a 5 gallon bucket filter, a 375 gph pump, and microfil filter material, bricks, 12"x12" ceramic tile, and 1/4 inch steel wire.
Step 1: I made sure that there was only one good entry point on the pond by placing it in the corner of my yard (this way i spend less on shorter pond fencing) and tilting the pond in the ground - seems wrong, but it works so right - about 5 inches lower on the entry point side. This also keeps the ducks from exiting the pond in any area with new or delicate vegetation.
Step 2: put together the filter and aquaponic setup
Step 3: determined where the water pump would go (farthest point away from the water inlet). Elevated water pump with 1/4 inch wire handmade stand.)
Step 4: I fenced the other areas so that I could start putting potted and planted vegetation around the pond without the ducks immediately decimating it.
Step 5, placed ceramic tiles on top of bricks at the bottom of the pool in a way where no matter where they move, they will be supported by brick (so that they cannot fall down and smash the fish.
Step 6: then i filled the pond, tested the pump and addressed the initial leaks.
Step 7: put plants in to help filter the water in the aquaponics and put plants into the containers to grow and feed the ducks with
Step 8: placed goldfish into the pond and let nature take its course. a few goldfish and a bunch of minnows got eaten right away but then about 3/4 of the whole group is pretty smart so they survived. I now have more fish than I started out with.
I am pretty sure that I will have to take out the gold fish when they reach about 7 inches because they won't really fit comfortably under the brick and will need a bigger container, inaccessible to the ducks, but that won't be for a good while. Until then, I think that these goldfish will have a decent time in here.