I think coral rubble is good but not done the way I first did, trying to give the large smasher as much "tools" to build with, the fact is a simple and sad one that it's nigh impossible to give a large O.scyllarus all it needs to build a burrow it would in the wild...
Their burrows in the wild can basically be limitless in size and a never ending build, I saw a burrow of an O.scyllarus on Nat Geo larger then a diver who sat down beside it on the sea floor, there is no way we can provide that and the O.scyllarus just seemed to grab everything not fixed down and stack it on top of its burrow like a pile.
My new approach is similar but more understanding.
I use the same substrate as I have always for these smashers, large dead chunks of coral, however I only place in enough to jusssssssst cover the base of the tank, it's more for aesthetically pleasing the eye as I don't like bare bottom, however it still allows the Stomatopod to gather up useful pieces of rubble to use for the entrance/exit to the burrows I provide, I just replace what they took from the base with a few more pieces of rubble to hide the tanks bottom.
By doing this I have made it really easy to simply syphon over the substrate and suck up all nasty to get it clean and sparkly again

in my smallest system STAND A I actually syphon the gunk into the Refugium in the last compartment, spraying all the 'waste' over the macro algae, seems to like it and it's even easier

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