I decided to run a little trial on this.
-Steve Morse
I ran a second trial after seeing a post on an aquarium discussion board that said that Seachem's "Purigen" filtration media would remove tannins. Unfortunately I lost my link to that post, so I can't pass it on. Once again, I boiled up a nice "tea" of (mostly oak this time) leaves from my yard, and water (first picture). Then after letting it cool, I saved one glass as a control, and mixed a second batch with about 90 g of Purigen aquarium filter media (second picture) and let it sit with an occasional shake of the jar for about 20 hours. Then it was strained through a coffee filter and put in a glass for comparison (third picture). The control is the glass on the left, and the Purigen treatment is the glass on the right. Purigen did lighten the color significantly although it did not eliminate it. It seemed to cause precipitation or flocculation of a dark colored material.
I have an interest in removing tannins because I would like to be able to crate a setup with an unconsolidated organic substrate. In my attempts so far I have had some technical success, but have not been able to see the animals as well as I would like because the water was just too stained. It seems that Purigen might be worth a try at least in an actual aquarium setup. If I do this I will report.