KKFok: Now that you've decided on Fakarava, I would wholeheartedly recommend that you dive with Serge and Carinne at Fakarava Diving Centre (FDC). Google it. I dove with them the two times I was in Fakarava. I also dove once with Te Ava Nui (TAN) one afternoon when Serge was all booked, and in terms of service, I like Serge a lot better.
That is not to say that TAN is bad, but diving with FDC is a lot more personalized - small boat (6-8 people max) - we never dove with more than 6 total. You drop off your gear and you never touch it again until the day you leave. And they really wash it after every day's diving. At TAN, it's a larger operation and it felt kind of crowded at the shop. Plus, you had to don your BC and tank and carry it out to the boat.
Serge gives excellent dive briefings and treats you like a responsible diver.
He also offers trips to the southern pass (it does cost more) and to me, the southern pass is better than the northern because, as I said before, it's narrower and the sharks are less scattered. It's a 2+ hour ride down and the tides have to be right but at least when I went, it was really worth it, and Serge does a really good one-day trip down there with two dives with a lunch on one of the small sandy motus.
Only other place where the grey reefs are even more densely packed is Apataki Atoll - as Mike V. can attest - but that could only be dived when the Aggressor was still in the Tuamotos.
Also, as Mike V. suggested, Toau Atoll is a great atoll to dive at, also with two separate passes. Serge does trips there if there is enough demand.
May I also suggest the bed and breakfast where I stayed: it's called Tokerau and it's about a 5 minute walk south of the Maitai Hotel. It is run by a Mme Flora and her daughter Gahina. Really nice people, decent food, and the bungalows are well maintained and, in my opinion, the nicest amongst the various bed & breakfast pensions. Carinne can make the arrangements for you.
Unless you stay at the Maitai (which, I was told, doesn't have mosquitoes but will still make you bleed from the price), I strongly recommend that you bring a mosquito net. You can hang it by tying a strand of dental floss from window to window.
Mike V.: you are absolutely right. Wrasse Hole is the name that the Aggressor crew used for that area in the northern pass, altho the rationale behind "wrasse" eludes me. The place is loaded with clouds of Humpback Snappers, Bigeyes (the red fish in Mike V's pic), bannerfish, squirrelfish.....just about everything except large number of wrasses. Unless they were referring to the Napoleons.
In any case, "The Valley" or La Vallee is what Serge called it, and being the senile old fart that I am, that's what came to mind first.