Digging for China

So I get my female clown digging out a nesting area, but I have a big hole forming in my big tank that had me stumped. I had never caught anyone in the act, nor did anyone appear to use it for sleeping. Today, I catch the Tang and the Butterfly fish working together to continue the dig. I guess they think they can make little ButterTangs together.
 
If you get "buttertang" fry. Put me in line for a buy! LoL. What's the current like in your tank? Specifically where they are digging.
 
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