@callingcolleen1 -- I think you over-estimate the power of caterpillar dung. For one thing, I do not have the town-swallowing hordes of caterpillars that you apparently have. In nearly 30 years of living in Colorado, I've maybe seen a dozen here in town. To have enough caterpillars here to turn my water a brownish color, I would need to have hundreds, if not thousands of them in my tree. But I do not.
What is really happening is that the dead leave sit in the bottom of the pond through the Winter, and when the water starts to warm up in the Spring, nature begins to do its job of decomposing the dead matter. That is when the tannins are released, giving the water its brownish 'tea' color. During the Winter months, there is a very slow process of decomposition which releases toxins in the water that can be deadly to fish if enough build up. This is one of the reasons why it is important to keep an air-hole through the ice during the Winter -- it not only allows the exchange of oxygen into the water, but also allows other gases to escape from the water.
You keep mentioning how the process of dead leaves and other waste in the water is completely natural; that in nature the fish do not die from all the leaves sitting in the water over the Winter. I would guess you have never been around a body of water that is completely enclosed, filled only by rainwater and snow runoff? You've never smelled the stench of the algae and decomposing plants that died in this water? And you've never noticed that there are no fish alive in these conditions - only a few select species (mostly insects) that can actually survive. You have completely missed an extremely important difference between a natural pond and our little water gardens... A natural pond is continuously being fed new water by a river... This running water carries away much of the dead organic matter, and the motion of the water helps break down that organic matter much faster than if it simply sits in the bottom and rots away. Compare that to our garden ponds, which are completely self-contained. Even with good filtration and continuously running pumps, you are still just circulating the same water through your pond, so any contaminants will continue to build up. During the Winter months, you do not have much of any plant activity to dispose of these contaminants, and there is no fresh water coming in to flush out the pond.
You may have fish that are 20 years old, but based on what you have been saying here, I can only assume that you have a bad misunderstanding of how the natural process works, or you have managed to imply exactly the opposite of what you actually mean. Regardless, simply based on the fact that I scoop the leaves out of my pond in the Fall, and you do not - I would bet on the health of my fish over your's any time. If you want to mimic what nature does, then you have to get rid of the excess decaying organic matter. A little won't hurt, but a lot can kill.
