plenums and digging creatures

[somethings fishy]

I recently picked up a yellow head striped goby and the LFS offered to give me a Goby shrimp that they had found in the deal saying they liked the companionship of each other. anyways for unkown reasons the goby died of unknown causes and this shrimp is digging large tunnels all through my DSB. I can see it through the side of my tank. I have a 4in. SB with a 1.5in. plenum I do have a screen layer 2in. below the top layer of the DSB to keep critters from digging, but this shrimp is managing to get below it from the side. My worry is that he is going to mess up all that wonderful chemistry in the lower level of the DSB by making those tunnels and probably introducing oxygen in the anaerobic layer making it aerobic?(might have these reversed).Any of you plenum experts out there got any advice. I fear that this shrimp will need to be removed which would be no easy task.