Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Just about anything that is composted is a possible source of humus.
Usually a compost pile starts as chipped twigs, grass trimmings, leaves (shredded or whole), plant based kitchen waste and so on.
It gets moistened and turned regularly for the optimum microorganism activity.
The end result is usually dark, fine, loose material. The last few bits of twigs might still be visible, but in finished compost you should not be able to ID what it used to be.
I would be concerned that the stuff you found (old aquarium plants and dead snails) might not have fully composted. If it looks like fine crumbly soil, then it is done. If you can still tell that it used to be plants, and still see snail shells, then I would add this to a proper compost pile, not use it the way it is.