old thread i know.
various snails, various worms, fungus's and molds, don't forget the bacteria.
yes there is a freshwater aquatic mushroom, doubt you'll find it in the hobby, but it exists.
mixing burrowing snails and plants, the plant roots will win and the snails will eventually be forced to the surface as the plant roots make burrowing for these snails impossible.
various aquatic worms are hard to find, tubifex worms and blackworms are the only ones i have heard of advertised in the hobby, tubifex worms will likely be eaten as they stick their buts up out of the substrate and wiggle it around to breath, i'm sure fish are more than tempted already, the extra action is probably more than they need to incite them to eat. blackworms will eventually all be eaten but not soon enough is my guess if you are worried
other aquatic worms, ... at least for the substrate, much searching has not yielded much for results.
try looking up aufshich, as for any specific organisms that compose aufchich, i have only been able to find they call it aufshich, ... a fine layer on the surface of the substrate composed of tiny organisms and algae that various critters eat - usually one part or another of the aufshich, not all of it, they sift through it to find whatever their dietary requirements are.
other fungus's and molds, ... we are raised to believe the existance of these are signs of disaster waiting to happen, ... so not much is known about actual safety or safe species to obtain, or how to obtain any desired species at all.
all these are decomposers, not all are available or even known in the hobby
concerns about snails eating, and pooping again so they are not efficient decomposers, ... sure they poop, but it's farther broken down, this in turn saves it from contributing as much to your ammonia levels and in healthier for your tank.
some view snails as pests, claiming they eat their plants, almost guaranteed snails are never going to eat your healthy plants, they may eat from parts that have decomposed some due to those leaves having various deficiencies, - don't blame your snails
some start feeding extra so the snails don't starve, this is a bad practice, feed your fish, the snails will eat the extra, but otherwise ignore the snails for feeding, just feed your fish
Edit:
found proper spelling
"aufwuchs"
as mentioned above, i am still looking into this as it is more described as a catch-all without any specifics