Very nice job on the pond. I like the plants in and around it but can understand the hassle keeping it from becoming overgrown. As for the fish thieves, around here (California) people use painted wooden owls, but any life sized, lifelike replica of a predator they would have cause to fear should do the same job. Perhaps you could pick up a couple of cat statues to place near the pond and/or a replica of some kind of avian predator indiginous to your area that would keep them away?