Now I have been using oak leaves for years, from different oaks. This year I put a large stock since I had too little last year.
I have tried many different leaves, but I keep leaves in the water for almost a year, which does not mean I leave the leaves for a year in the aquarium, but they stay together when I remove them, other leaves are already so decomposed after a few weeks They are only sludge and pollute the water. I prefer the leaves of the marshes, they have the largest leaves and I need 4 or 5 leaves per aquarium.