From The Aquarium Wiki
Dried Oak leaves can be used in aquariums to add tannins and essential minerals to aid the health of your aquatic animals and plants.
If used in large amounts they will also reduce the pH of the water.
It also suppresses bad bacteria and fungus and aids the development of fish fry and tadpoles by promoting infusoria growth.
- Used by many Killi fish keepers.
Only collect dry leaves during autumn/early winter that have naturally fallen off or just about to. Keep the leaves in a air proof bag and they will keep for many months.
- Do not collect leaves near a road or at the edge of a farmer field as it may have chemicals on it.
- Do not collect leaves with bird droppings on them.
Some people don't like the tannins given off by Oak leaves, so boil them in water to hasten their release or use less leaves.
- This boiling process destroys the naturally good bacteria, fungus and infusoria on them.
So if you don't want the tannins, but do wish the health benefits, use less leaves.
- Rinse the leaf under tap water to remove any possible containments.
About 0.5-1 good sized dried leaf per 50 Litres (13.2 US G.) every 2 weeks.