Helfrichi's Firefish digging a pit in the sand. Why?

[GroktheCube]

For the past few days, I've been observing a very odd behavior in my firefish. At first, I was concerned that it might have acquired some sort of parasite like ich (though all fish are quarantined and treated prophylactically via ttm), and is was repeatedly diving towards the sand, and then turning up at the last moment so that only the area right before the tail made contact with the skin. Over time, this ended up creating a marked indentation in the sand.

Today, it has started hovering over the pit, then backing up towards it, and rapidly moving its tail to fan more sand out without actually making contact with the sand. It doesn't do it constantly, but it will spend 30 seconds or so excavating every 10-20 minutes.

His behavior is otherwise normal. Breathing normal, color normal, mucous coating normal, fins normal, appetite normal. I have absolutely no idea what it is doing, but it appears like it is intentionally trying to build a pit in the sand. Has anyone observed this behavior before?

Tank is a 120g mixed reef, the firefish was the first fish in the tank some 9 months ago. Other inhabitants are a pair of McCosker's flasher wrasses, a tailspot blenny, and a twinspot bristletooth tang. The last fish in were the wrasses, added about a bit more than a month ago.

Water chemistry:
Salinity: 35ppt
Temp: 78
pH: 8.1-8.28
Alk: ~7.5k dKH
Cal: 480
Mag: 1350
NO3: 0
PO4: ~6 ppb

 

[MikeandNicole]

Held riches want a bolt hole to disappear to when they are scared. This is normal behavior and good because the other option is jumping out of your tank.

 

[Spyderturbo007]

Totally normal. I think that's part of the reason they are called Dart Fish. Well, that and they are incredibility fast.

They will dig a hole and then hang out by that hole until they feel threatened. At that point they will fly into that hole so fast you won't know what happened.