We live in an older urban area in Tampa, Florida's Interbay Peninsula. There is a drainage ditch at the back of the property and enough cover for raccoons, possum, and several generations of feral cats.
We have a koi pond, a filter pond with plants and shibunken, and three smaller "kettle" sites. The kettles are screened vertically to a height of about two feet above the water.
Our worst predation has come from a great blue heron (the heron from hell) whose territorial instinct picks up in the spring. He generally does not approach the house except to stalk about the roof waiting for a chance when the labradors have gotten bored. A smaller Louisiana night heron will come up to the house where two of the kettles are but so far the screening has kept him from feeding on the comets. The koi, like the comets, are fast and wary enough to have mostly escaped being dinner.
One of our kettles is in an enclosed 12x12 vine covered space off the dining room and on the street side of the house. The feral cats are often prowling there and even have nested in the vines. It was there that the latest predation happened and it has us baffled.
That kettle (probably 20 gallons or so) held two fat fantails and two fat black moors, all probably eight inches long. Overnight, all these very slow(in all senses of the word) fish were killed. Three of them had been bitten in the head and then left floating. The fourth had been eaten down to the mid-body, head and all. In addition, a fair-sized bullfrog was struggling in the water. His right hind leg had been bitten off at the hip.
My first guess is a raccoon, but I do not understand how one would approach from the street side. We have often seen them in the back and one evening two cubs were treed outside our fence by the dogs. I am told that possum generally leave the heads and eat everything else. Also, I have never heard of a possum cleaning out all the fish this way, much less being able to catch and maim a frog. As for the cats, whatever did it had to climb a screen and enter the water to do this damage and I cannot see a cat doing that.
So, my questions are: What would get into the water to do this? What would bite fish on the head to kill them and then jettison them? What could catch and then bite off the leg of a bullfrog?
I am still guessing raccoon, but would appreciate any ideas.
BenOrr

I've not known opossums to be big fish killers. Raccoons and feral cats are certainly possibilities. The raccoons are going to be the best at fishing. I'm confused on the setup. Why wouldn't a raccoon cross the street? Why wouldn't a cat climb a screen if a raccoon could? Oh, you mean the killer had to actually be in the water? You're right there; cats don't like getting wet. So, the raccoons may be responsible. Our raccoons pull out snails and then don't even eat them, leaving the poor things to dessicate or freeze in the winter. They don't get our fish though but my pond is 1800 gallons. You may need to cover the small ponds with netting. My pond is covered with netting to keep the herons out. I hate it but it works the best.

Someone gave me a miniature mirrored disco ball which I hung for a joke on my front porch where it's sunny in the morning. Great lights all around! But no birds. I thought, hmmm. Might that keep birds away from a pond? I couldn't get the ball to spin near the pond, so I gave up and forgot about it. But one day I put one of those shiny glass gazing balls into the pond and let it float about just for the heck of it. If you fill it about 1//3 full of water and it won't go crazy. Haven't had any annoying birds there--ever. That was just a happy accident; it's amusing to watch the ball float about and reflect the pondscape. So maybe it's a good egret deterrent in ponds and yards, too?