I don't currently have access to almond leaves, so I tossed a few sterilized leaves I have left over from insect keeping on to the top of the water. Both of my bettas seem to like them, and the male promptly used them as an anchor for a bubble nest.
I am assuming they are safe as more than a day has passed with no negative effects, but thought I would check. (And if they were harmful in some way they wouldn't be sold for use with living creatures.)
It would be my luck though to have them be the one kind of leaf that is dangerous to bettas... So is there any type of leaf that is flat out bad for them?