You know that fish need well-oxygenated water to survive. But did you know that low oxygen levels aren't your fishes’ only nemeses; other things can harm your fish, as well. And they do so with much more cunning.

One summer’s morning, after going outside to check on our ponds and to feed our fish, we noticed that some water lilies in the lower pond had been knocked over. Upon closer inspection, we found some of the leaves shredded and some of the rocks on the spillway upset. There was no sign of 13 small goldfish that had occupied the pond--only a few glimmering scales on the top of the pond wall.

Raccoons are notorious water scavengers. They love rooting around shallow streams and ponds in search of a good meal. Since they’re omnivorous, fish make an excellent target, particularly so in a small pond.

A few nights later, they raided our lily barrel, as well, and stole all but one fish from it. We assumed that, since the barrel was nearly two feet deep, the fish inside would be safe. We were wrong. We named the solitary survivor "Lucky" and moved him—as traumatized as he was—to a deeper pond with a large rock ledge under which our fish can hide from predators.

A raised rock on the bottom of a pond serves more than one purpose. Besides offering fish protection from predators, it provides a place for skittish fish to hang out.

We bought a small silver koi measuring about 4 inches several years ago. We brought him home and introduced him into a pond just above our large holding pond. We were confident he wouldn't go over the falls joining the two.

Naturally, he went over the falls, and we found him—somewhat traumatized—swimming around the big pond with our monster goldfish the next morning. He soon took to hiding out in the skimmer box during the day and coming out to forage for food toward night. Then he discovered a couple of medium-sized stones near the base of the falls. The stones formed a natural “tunnel” into which Casper could dart for cover whenever he felt threatened.

There are many threats to pond fish, both man-made and natural. Since our first two raccoon attacks, we critter-proofed all of our water features, making sure that the shallower ones have plenty of deep rock ledges under which the fish can go for protection. As for the wine barrel, we fitted it with a slatted wooden top that we place over the opening every evening so that nothing can get in.

Although the Great Coon Attack of 2004 was the single greatest loss of fish in our fish-keeping history, it may not be the last. We recently ran into an elderly couple who told us that they had come home one afternoon, gone out back to visit their pond, and noticed a Great Blue Heron nesting above their garage. It returned, much to their delight, daily.

All went well until the couple walked out to their pond one day and happened to catch sight of the bird just as it was flying off. It did not leave hungry. It had emptied the pond of their prized koi!

What other raiders of the dark might you have to worry about? I’m pretty sure opossum will eat fish, if they can catch them. Cats might try snagging one now and again if they're not declawed. Some species of diving waterfowl might find happy hunting in your water garden.

Of course, you can shag away potential predators or scare them off with a loud cap pistol or a whistle, if and when you find them. But, it’s best to provide your fish with some means of escape from a direct attack, such as a large flat piece of flagstone raised 4 inches (or 5 or 6, as necessary) off the pond floor and placed in the deepest part of the pool. In that way, if something does reach in or dive in or do whatever it does in the wild to catch its food, the fish—with their amazing quickness and instinct for survival—will be able to duck out of reach and remain there until the coast is clear.