Leaf litter

Leaf litter doesn't last long on the floor of a rainforest. But in a monsoon climate, masses of leaves drop in sync with the dry season, and some make their way into streams. Natural leaf litter in a streambed forms a connected network of small hidden tunnel-like spaces. In leaf litter up to a meter deep, Apistogramma, for example, live and breed, sometimes in unexpected densities. Large surface area and a wide scale of spaces offer a habitat for a widely-diverse biofilm.
 
In temperate waters, which are more thoroughly studied than tropical waters, "aggregations of leaves on the stream bottom usually support the greatest diversity and abundance of invertebrates, and the addition of leaves to a mineral substrate results in higher densities of animals" reports S. M. Mandaville, writing about the "Zoobenthos of freshwaters". 
 
Dead, dried leaves will eventually become skeletonized and decay, and perhaps they should be removed and replaced. I never do. But unlike detritus formed from the soft living leaf, dead leaves have fewer nitrogenous compounds to release into the water. I get a couple of months out of cutleaf  beech leaves, before they are reduced to fragments of skeleton and brown flakes, but other leaves, like some viburnums, are pale skeletons in a matter of days.
 
Why are dried leaves okay in the aquarium, whereas softening aquatic plants release nutrients that make problems? As a leaf on a deciduous tree or shrub cuts down its photosynthesis in fall or at the onset of the dry season, other processes are under way. Metabolic pathways start to fail. Compounds degrade and break apart. Doomed leaf cells salvage the valuables, especially nitrogen, by sending them off to safer storage tissue. Finally, a corky abcission layer seals off the leaf at the base of its stalk, and the dry leafy skeleton falls away.
 
Not all dead dried leaves will be suitable. Oak leaves, for example, contain an unusually high concentration of tannins that could be mildly toxic if you used too many.  — No, I don't know how many that would be. Magnolia and rhododendron leaves are possibilities I haven't tried; would they leach toxic metabolites? But their leathery texture would make them ideal.
 
Beech leaves are the tried-and-true leaf-litter elements in European "natural" aquaria. They do leach tannins and other weak humic substances that will acidify very soft water (dGH below 5), but they won't have any noticeable effect in moderately buffered waters. If I steep beech leaves first in boiling water, they release a delicate odor, like the linden-flower tisane that unleashed for Marcel Proust the remembrance of things past. Beeches hold their brown leaves quite late into the winter. In beech country you can conveniently pick a large boxful of leaves and store them, as long as they're perfectly dry. (Pasta boxes even have cellophane windows.) If they aren't matted down, and haven't been skeletonized by minute insects yet, you can even use the fallen leaves. The insects and mites that inhabit all leaf litter won't escape to run around your house (an uninviting habitat for them), but you might want to put the fallen leaves right into the aquarium, where your smaller fishes will eagerly pounce on any mites or springtails.
 
Of all beeches, the cutleaf European beech has the finest-textured leaf. It's a much smaller leaf than the ordinary beech leaf, with a waved serrated edge that keeps it from lying down too flat in the aquarium. I mention it because it's my favorite. At the end of October I cut a curious figure in Central Park, kneeling to carefully scoop unbroken dry cutleaf beech leaves into empty pasta boxes.
 
I've always avoided black walnut leaves because of the allelopathic substance (juglone) that suppresses grass and other green growth under black walnuts. It couldn't be good in the aquarium. I've never tried Japanese maple leaves: their small scale and deeply cut palmate shapes and russet color could be good.
 
Aquarists who are adding a dry leaf of the Indian Almond, Terminalia catappa, to their aquaria aren't doing so, by and large, for aesthetics but as a herbal remedy. Still, I notice at AquaBid that imperfect examples of these gigantic leaves bring a considerably lower price.
 
Links. Of course at  The Krib there are a series of postings about leaf litter, especially in reference to Apistos.