anonapersona, yes! In Houston I think your winters are mild enough for Melanoides to become a feral pest. They started in San Antonio and are spreading, especially southwards into Mexico. A major pest. I have some links to better-informed-than-me sites at
www.skepticalaquarist.com (Invertebrates: Snails) Check em out. I don't know how I'd deal with this issue myself. How can you avoid flushing a minute snail here or there?
Stuart, as detritus-eaters deal with recycling "waste"and get the goodness out of it at each stage-- catfish: snail: worm: ciliate: bacteria/fungi-- it becomes increasingly mineralized, ending up finally as metabolic water, carbon dioxide and some less degradeable organics that go to make "humus."
Funky as they are, feces are never toxic, though organisms do usually clear toxins one way or another, thus there will be some toxins in feces as there are in food..."Toxic" substances always are interfering in some way with some necessary biological pathway. Most organisms have techniques for dealing with toxic molecules that can't be broken down: such toxins often lodge in liver or kidney tissue, or in fat, if they can't be excreted.
As detritus is passed along and down, from scavenger to shredder to absorber, some potential energy is lost at each transfer from trophic level to trophic level. So there is less remaining to "rot" and "decay" in the bacterial/fungal community, after each intermediate detritivore has a go.
Of course snail feces are vivid against the white gravel. But snails don't
produce feces out of nothing. No actual stuff is
generated in the snail, only processed and broken down.
It's an important concept that affects a lot of how we think of these simplified captive ecosystems we're managing...
(
please excuse the geekatoid wonkish detail)
This is just my two cents and my experience on this ?