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One of my 3 blue-green chromis has spent at least the last half hour (and is still at the time of writing) digging in the sand. These fish have always scarred me a little with their intelligence, I have seen them learn from my goby, by kicking up sand and eating bits that come up, but today, one is "digging".... He/She is going to the bottom of the tank and leaning at about 45 degrees, then flapping all it's fins and body to stir up the sand. It's dug up a trench/pit about 6" long by 4-5" wide and about 2" deep so far. None of the other two chromis are near by... and it stops digging if larger fish come by, or if I move too close to the tank.
I've made a little low quality video from my digital camera if anyone's interested, it's rather strange behaviour... Is it trying to prepare to spawn/prepare a nest or something? It's actually chosen about the largest part of open sand and most exposed part of the tank, so maybe not. It's still digging and really begning to look like a pit/nest.... hmmmmm one of the other chromis just came up and made a funny gesture, sort of leaning and wiggleing towards the other one but was quickly pushed off and the other got back to digging... The chromis is really digging quite violently now...
Any ideas anyone?

I have no idea. My blue Damsel was doing the EXACT same thing a while back..

It's really weird, she's *Still* doing it now, there's quite a sizeable "crater" type hole there, sort of oval shape, 6-7" long by 4-5" wide.
If anyone's interested:

http://www.reptaquarium.co.uk/photos/chromis%20digging%201.avi
http://www.reptaquarium.co.uk/photos/chromis%20digging%202.avi

I appologise for the low quality, shame my camera can't film in 3MP as well =D.

[span class="edited"][Edited by worley 2004-08-19 15:21][/span]

My skunk clonws dig holes and sleep in them. I doubt this is why it's being done, but I'm not positive.

I have seen many species of fish dig or "slap" the sand in a show of dominance. Maybe this is the case?

I don't think it's a show, the chromis dug for around 2-3 hours and apears to have stopped now before the white lights went off. They tend to swim around together and they are rarely agressive to each other (where a little when they started 8 months ago).
And now my shark is sulking and doesn't want to eat tonight lol.

It's probably a Male trying to dig a spawning pit. Your doing something right if you can keep a cleaner wrasse alive for more than a month, tanks looks good...I kind of miss my salt tank(s).

Ah could be!
I've had the cleaner wrasse for around 6-7 months now, he accepts *any* food now, brine shrimp, mysis, krill, even flakes. He was about 2 minutes away from dieing of whitespot about a week after I got him, but I dipped him in Kent Marine RXP, and he went from almost not breathing/moving at all to swimming around in just 5 minutes of dipping, and has been great ever since. He happily cleans the yellow tang, but ignores all the others (including the bannerfish).