I have a huge Magnolia treet in my yard, but never considered harvesting the leaves. A few weeks back I had to cut back many of the branches from blocking the sidewalk, would've been a great opportunity, but unfortunately they were already picked up as yard refuse. Huge tree though, so will be tons of leaves available in the near future. We don't live close enough to anyone that would be spraying for mosquitoes, so I'm sure its safe, but would wash them regardless. I'll take a picture of a leaf later as I know nothing about trees and how to identify them, and don't trust google for images. I'll post when I can, if motivated enough I'll collect some to bring in, I don't think I really have any fish that would benefit from or appreciate leaf litter, plus I'd likely be lazy and end up with dirted tanks. Interesting topic though, I know I'd never buy leaves.