In my experience with cats inside and outside… they have never tried to get to the pond fish. Water is deep enough and the fish hide in cracks / crevices in rocks in the pond. A small water fall helps the water move to improve oxygen content in the water and a bio filter keeps the water stay clean. I don’t recommend fish in a wheel barrel lined with pond water unless they are small and yor muders like gold fish that are air suckers and don’t require air in the water. Koy fish are a bit spendy. There is a water ratio per number of fish per gallon of water for ponds. I had several for years in my in ground pond . There are water plants that float on top of the water as well as plant roots allow areas for fish to stay safe down in the water. There are predators around here that are far more aggressive than kitty cats. For instance the Giant blue heron and the raccoons are more of a threat to the pond fish than any cats around here and there are many feral cats here . Never had a cat explore the pond for fish. I have many cats.
Thank you for the knee surgery update. I am ‘Happy’ to hear your on the mend and slowly improving since knee surgery. Knee replacement surgery isn’t for sissies that’s for sure. You are a tough gal staying in the healing groove and sending hugs and healing blessings your way.