Tannins are released from decaying leaves, pine needles and the like in the pond water. The first thing to do is remove as much of the decaying plant material as you can from the bottom of the pond. This will not solve the problem, but it will help stop it from getting worse.
The next thing you need to do is make some fairly big water changes. Do this over the course of several weeks, so as not to stress your fish out TOO much. Tannins are toxic to fish, so they are already stressed and it is important that you remove them. The tannins, that is. Not the fish.
If you STILL have brown water after doing three or four major water changes, you may need to resort to a mechanical means of removing them.
Two ways to remove tannins from your pond water are by 1. Utilizing Activated Filter Carbon, and 2. Installing a UV clarifier. Both must be IN the direct stream of the water flow.
A UV clarifier is a device you install in the plumbing between your skimmer box and filter box. It has a little ultra violet (hence the term "UV") light bulb that zaps anything that passes by it, and I mean ANYTHING. We're talking tannins, suspended algae, good and bad bacteria and parasites. A UV clarifier is a good thing to have, but until recently, they were very expensive, and the bulb was costly to replace. Pondmaster has recently introduced a submersible UV clarifier, which is reasonably priced. Now that market demand has driven the price of UV clarifiers to something that is within reason, I will be adding a section on UV clarifiers to the list of products we carry. Please check back for descriptions and pricing. If you can't wait until it's posted on the main web site, write me and I'll get a quote right out to you.
First though, do your water changes and call me in the morning.