Welcome, Nick! When you said your pond is "brackish", that mean a high salt content, and it may be possible there is a high saline level considering your location. If Tannins are the culprit for the brown staining, about all you can do is identify, remove and replace the offending plant life that is causing the staining. You could filter the water, but that would be a pretty pricey endeavor. I am guessing the "sludge" you have on the bottom is from the very steep sides sloughing off. I would bet you soil is mostly sand and decayed leaf litter with all that lush vegetation in the pictures.
The tannins themselves are not harmful, but the acidic environment that usually leaches the tannins out of plant life is often very hard on fish. Do a pH test first, or get your water sampled as mentioned earlier. If you can get Ag Lime or bags of pelleted lime, that would be probably be a huge benefit. Clearing some of the trees from the edges and replacing with a grass fliter will help a lot also!
Last edited by Rainman; 03/08/15 09:31 AM.
