This is a blue-lined sea bream (Symphorichthys spilurus), also known as a sailfin snapper. It's a beautiful fish.
They are reported to frequent sandy-bottom areas around coral reefs, in the five to 60m range, but in over 20 years of diving, I've only ever seen them a couple of times, maybe.
They do gather in large numbers for spawning though. And by large numbers, I mean like this:
I've been on a quest for a couple of years now to photograph the actual spawning. Unfortunately, it's super-difficult. Sigh.
Spawning takes place at depth...like, 35m is shallow (which means there's not a lot of light)...and usually in strong current...like, get swept away in the blink-of-an-eye type current.
To make it even more difficult, the actual act of spawning only involves a handful of fish at a time...even though the gathered school comprises thousands of individuals...and takes place so fast that lightning seems slow by comparison. Sigh again.
This is at 10 hours post-fertilisation:
And this at about 35 hours:
Glutton for punishment that I am, I'm sure I'll go back to Palau to try to photograph the actual spawning again.