We had a very similar sitaution with raccoons! We have have a large preformed pond, and in the past had never had any problems with predators. Only occaisonal geese hanging out and making a mess. We came out one morning to find all of our potted water lillies missing and the pond a muddy mess.
It turns out the racoons were using them to stand on so they could grab fish.And the pots were knocked off of the "plant shelves" and into the deeper portion. We lost several medium size koi over 3 consecutive nights. Even without the plants on them the plant shelves basically gave the raccoons a perfect fishing spot. So they may have actually just knocked them off to make things easier.
We went to the local garden center and purchased a mesh pond net to go over the top and placed several large pond rocks around the perimeter to keep it secure and taut over the top of the pond. The first night there were no problems, but a few days later we came out to find the net half off and in the water, several of the rocks in the bottom of the pond, and the potted plants knocked off again. But luckily no missing fish.
We secured it under more of the surrounding rocks and the next morning came out to find the net chewed or ripped through and 1 large koi half eaten on the lawn a few feet away from the pond. So we went to another garden center to find a different racoon-proof option. It turned out they had recently had major raccoon problems themseves despite being in a much more urban area, and the mesh nets or metal chicken wire didnt stop them either.
They told us that they had been so frustrated from so many missing fisht that they were tempted to use live traps and take the raccoons them "somewhere" from where they would never come back if you catch my drift. Instead they said they used alligator decoys which they normally to sold to scare ducks and geese away from pools and ponds. Similar to these:
http://www.gardenandpatiohomeg... And they worked for the raccoons too even though we don't have alligators around here.
We went with one with glowing eyes for extra emphasis, and since we've added "Mr. Toothy" as our daugher calls him, no more raccoon problems. We also haven't had and ducks or geese either. He has worked well since we didn't like the idea of a net or anything else blocking the view of our pond so much.
To comment on the oak leaves mentioned above, I've heard they are dangerous too, but we have a ton of oak leaves in our pond every fall from an adjacent tree and never have had a problem yet. So it might be a myth.
