I feed my pond goldfish every other day, and part of what they get is zucchini. I blanch zucchini, then freeze it, and thaw what I want to use with boiling water. I use metal skewers that hang on the side of the pond (mine is an above ground stock tank, six years old now).
Goldfish are omnivores, but I think giving them veggies helps to prevent bloat and swim bladder problems. They love the zucchini and a couple of them will actually jump out of the water a few inches when I reach in to remove the old skewer and put the new one on.
I've never had a problem with swim bladder or bloat in any of these fish. YMMV.