I've had lots of cats show interest in the goldfish. Only once had one snag a fish out of the water. The goldfish wiggled off the claw and was fine. I also have catfish in the pond. The black channel cats survived the heron, and grew to about 2 feet long, but succumbed to raccoons that swam to the bottom and proceeded to strew catfish parts all over the backyard. I was kinda glad they went. I didn't know they would get that big. There still may be more catfish in there just haven't seen them in a while. I also had Dojo loaches. They got big and survived at least 3 years, haven't seen them in about 5 years. They looked like foot long eels the last time I saw them. Friends told me to buy about 40 feeder goldfish first and expect them to all die, then in a couple weeks I could start adding the fish I wanted. Well the feeders mostly all survived and grew and bred. I've never had to buy more. I do have a couple Shebunkins in with them as well. None of the fancy goldfish ever survived. I think the catfish ate them, before they got theirs. Lol. Currently there are about 30-40 goldfish, and the pond is about 800 gallons. As far as algae, I mostly have an issue during early summer. The heatwave we had a couple weeks ago killed it all off. Crystal clear now, and throughout the winter.