I have a pond with one surviving goldfish about 6 inches long. I thought all my fish died in the heat wave two months ago except one. Then after a few days he disappeared too, I looked for him for weeks and nothing, so I assumed a raccoon got him and we have no more fish left.
I cleaned out the entire pond last weekend. When the water went down from 24" to about 8" I saw the fish hiding in some string algae and I caught him. So now he is living in a Home Depot 5 gallon bucket until I finish with the pond rework which will take a few weeks. I set up a small pump with some air stones for now.
The problem is food. I have never fed these goldfish, they lived off whatever is in the pond, bacteria, algae, tadpoles, worms, crabs, whatever else...I gave him some dry fish food and he doesn't touch them. I am not sure he knows that's food.