i remembered after you posted your first response that MTS don't deal well with roots, ... i'm going to assume the same is true of assassin snails.
wanting tasty worms, wanting a lack of digging & bottom dwelling fish, so those that do notice the worms 'shouldn't' eat them into extinction is my thinking. ... but still provide the tasty morsel if caught
Assassin snails burrow just like MTS, ... to burrow through roots though, ... root are quite the reinforcment for the substrate that the snails would have to pull their shells through (one of the things i've red) MTS don't burrow around roots, ... well "around" but not through.
Assassins are lazy burrowers from my observations . I have them in my 16 and they will follow food into the substrate but not far . I have lots if roots in that tank so perhaps that's why they never venture past the top 3/4" of the substrate .
there isn't much written on rooted tanks, ... all i've come across on burrowing critters (worms & snails) burrow only 3/4-1"
speculation is past this the O2 level at depths lower than this is lower than preferred, ... and i think that's a lot of substrate to pull a shell through (personal guess & opinion), ... but the same is said about worms in the substrate.
one forum a lady had a DSB in her planted tank, ... she had mentioned when she moved plants around sometimes there were worms wrapped up in the roots she pulled out.
i have red that plant roots will put O2 in the substrate, including a test where they covered the substrate with a plastic sheet & just enough of a hole for the plant to grow through, ... results mentioned a steady increase in O2 in the substrate
so the 3/4 - 1" depth is related to O2
plant roots can extend this depth for worms
back to non-rooted tanks, ... this burrowing depth makes sense when considering non planted DSBs as after 3" the O2 levels are too low to be of value and anoxic environments set in.
to me, it sounds like O2 can be used up really fast in the substrate (counting depth) if there is nothing to add O2 back, ... even burrowing critters can only do so much.
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