Diana K

I would suggest the leaf litter or the potting soil with no fertilizer, but sift it to remove the larger pieces. I saw the post about the worm castings, too. Go for it! A blend of organic matter sources might be better than concentrating on one particular material. Each will decay at a different rate, and each may bring slightly different nutrients to the soil blend.
You could use peat moss, too, a light dusting on the bottom of the tank, then add the soil. I have used peat moss in larger amounts and it is OK as far as pH and other parameters. But it is so light that anything disturbs it and it takes a while to settle. It does not really cloud the water, the pieces are too big for that, but there are thousands of little bits drifting around, then they settle all over the plant leaves and everything else in the tank. I worry about the fishes' gills in conditions like that. Like trying to breath in a dust storm.