Not an expert in this area, all I know is most comments I've seen on this recommend clean, dried leaves to avoid possible toxins in still green leaves, or molds, etc. on leaves from damp areas.
What I do know is that some tree leaves are toxic to fish (examples in this link). Not an expert here, either, as to which are safe and which are not. So as far as leaves I might collect myself I've basically stuck to clean, dry oak leaves for the most part and not had any problems, since (besides almond leaves) this is what I've most commonly seen recommended, though I'm open to trying something else if I learn others have already found them safe.
Thanks for your response and for that link. I know the leaves I picked are safe once dry (Tony has the same type of leaves with his apistos). I guess it makes sense to wait until they brown.
I've always used brown leaves. Seems to me that green ones would still have a lot of gooey bits and saps still running around in them...but this is just my guess....I have no proof that they are worse than brown ones. Matt C has been experimenting (with some success I believe) with using fallen magnolia leaves in his tanks.
It is true that I use magnolia leaves, with success. They are more typically available in the spring, however.
I don't use green leaves, for the reasons others have advanced. It's possible that once they've fallen they are safe, but why take the chance, especially now, when oak leaves are plentiful?
I collected a zip lock bag stuffed full of dried leaves last year and am using the last of them up now. Used them on two tanks. I added some water to the bag to keep them moist and helps them sink faster IMO