I use oak leaves now and then, for food for fry. Dried leaves placed into the aquarium will effuse a sort of slime (I can't see it) and Infusoria appear, and fry will greedily eat this as a first food. The nutritional value declines over a few weeks I believe, although the leaves will stay and very slowly decompose over months. My Farlowella vitatta fry have been grazing oak leaves and growing well for 3 months now. It is rather neat, the way they go for these; at each weekly water change I add 5-6 leaves, and remove the oldest, and within a few minutes of refilling the tank, the three fry are charging onto the new leaves and grazing like mad.
With some fish, thinking of certain South American dwarf cichlids, a litter of leaves is ideal. The fish will flip these over in search of food, and use them as spawning sites. Fry hide under the leaves. All this is natural behaviours in the wild.
As to which leaves are safe/unsafe. I have read that oak, beech and almond leaves are fine. I'm sure there are others, I just haven't come across them. I have an oak tree in my yard, so this supplies all I need; this autumn I have so far collected several hundred leaves. There are some that do contain toxins and should never be used, but off the top of my head I can't remember names. I just remember the safe ones and stay with those.
And I agree, that is a lovely video Mikaila
, very nice indeed.