WHY OAK LEAVES by Bill Vannerson
There have been frequent threads over the years as to why oak leaves are preferable to other speciesí leaves, especially maple leaves, for tank litter in Killie tanks. The book I'm currently reading sheds some light on the subject. The Vegetation of Wisconsin: An Ordination of Plant Communities by John T. Curtis, 1959, University Wisconsin Press, details the differences between the composition of Oak and Maple leaves and may help explain why oak leaves are more favorable to hobbyists in creating soft, acidic water and leaf litter habitats for our fish.
In describing the life cycle of the Sugar Maple, the dominant tree in the Southern Mesic Forest, Curtis notes: "Another effect of the leaves is bought about by their high content of basic nutrient elements at the time of leaf shedding. . . . the nutrients which have been pulled into the leaf by the transpiration stream during the summer remain there and are not withdrawn into the trunks as in the oaks and many other species. When the leaves are shed, they contain high levels of calcium, magnesium and potassium. The high base content of the maple leaves is correlated with the ease of breakdown by the millipedes, fungi and other soil biota." So it's the base elements in maple leaves that likely causes them to quickly degenerate as well as release elements that would cause the water to become harder more alkaline, or at least prevent it from become soft and acidic like oak leaves do.
The author goes on to say, "[Xeric forest soils] show a greater acidity and lower nutrient content than comparable figures for mesic forest soils. The acidity is apparently the result of organic compounds, especially tannic acids, produced by the oak leaves. The ash content of leaves of various species of oaks varies from 4.5 to 7 percent, in contrast to levels of 10 to 12 percent in sugar maple and basswood. Of this amount of ash, only about 1/3 is in the form of calcium and magnesium oxides, compared to 2/3 in the mesic trees. The low lime content is associated with low palatability by millipedes and other soil fauna and may be partially responsible for the low rate of decay, induced by other soil biota."
So oak leaves have more tannins, which makes them more acid than maple leaves. But they also have less ash composition, which means they cannot release chemicals that would make the water more alkaline, or at least neutral. The lower ash content may also make the leaves less tasty (palatable) and slower to decay, assuming that the aquatic biota that's responsible for leaf decay have similar tastes to forest millipedes!
I must caution that this material is forty years old. But it would stand to reason that these simple statements have remained basically true.