PolyFilters. PolyFilters adsorb and absorb a wide range of contaminants, including dissolved organic substances. They have been made since 1976 only by Poly-Bio-Marine Inc. Their website is a model of accessible scientific clarity.
PolyFilter is one of the few aquarium products that scarcely needs to be marketed with hype. Its single drawback is that it's more expensive than activated charcoal. Since the white spun polyester pad turns dark mahogany brown from organics it has sorbed, you can see when it's time to replace the pad. PolyFilter also selectively sorbs toxic ammonia at levels above 0.1 mg/L but doesn't adsorb the harmless ionized form, ammonium, which is used by plants. It adsorbs phosphates and some nitrates (in freshwater, though not if you were using it in saltwater). I keep PolyFilter pads constantly in my filtration. The matrix is availables in 12 inch by 12 inch sheets, which are less expensive per square inch than the more commonly available "filter sleeve" size: in my small operation, one, cut in various sizes, lasts me almost a year.