Osmunda fiber

Osmunda fiber. Osmunda fiber is nothing more than the matted woody root system built up by Osmunda ferns. Though fir bark and coconut fiber (coir) have replaced it among commericial orchid growers, Osmunda fiber is still used as a growing medium by orchid hobbyists, and so it's often available pre-bagged at a larger plant store or a high-end florist. If they don't have it in stock they can probably get it for you. It's expensive, but you don't need a lot.
 
Osmunda fiber isn't used nearly often enough in aquaria. Stored dry, it keeps for years in the bag it came in. In the aquarium it softens but doesn't break down for nearly a year.
 
Osmunda fiber is acidic. Osmunda fiber extracts in reverse osmosis or distilled water result in pH values down around pH 4.5-5.0, according to a CP mailing-list post by Jeff, NC, 31 Jan 1994. In buffered water, pH values won't drop as low as this.
 
Osmunda fiber turns a splendid blackish red-brown in the aquarium. It's full of tannins, which will leach into the water, imparting the golden color that makes tetras glow. Tannins and other humic substances give Osmunda fiber, like peat, some water-softening capacity. I've never tested Osmunda fiber against peat, dry weight for dry weight, to compare them in this respect. Another thing for you to do.
 
Wiry, springy Osmunda fiber creates vast surfaces for biofilm. If it traps too much detritus, you can pluck it out and swish it in de-chlorinated water to clean it. If it gets too covered with algae, a week in a bowl of water in a dark closet will eliminate the algae Before I use Osmunda fiber, I steep it overnight in water that's been brought to the boil. This makes the tangle of roots more flexible, so that I can tease them open a little. The resulting "tea" makes a home-brew "blackwater extract" guaranteed not to contain added salt or other spurious ingredients.
 
The loose tangle of Osmunda fiber provides refuge for fry, and for gammarids or daphnia. Shreds of Osmunda snagged in bogwood help form the complicated territory boundaries that make it possible to keep several small territorial fishes in a smaller tank: rivals hidden from sight are always less of a challenge.