Here's the finished product for the light fixture. (There's some artifacts in the photo that make the finish look worse than it is for some reason.)
I switched out the blue XP-Es I had been using. Now I'm using 2 dimmable Meanwell drivers to run:
10 cool white Cree XP-Gs
8 warm white Cree XP-Gs
and 9 neutral whte Cree XP-Gs.
I may put the blues back on at some later date, wired separately with their own driver.
I have the drivers set to deliver about 1300 mA at max, but with both strings running at that power it's just ridiculously bright. That just has to be way too much light, even with the fixture 9" from the top of the tank. Fortunately it is easy to dim.
The overall color balance looks good to me, and with so much headroom in the overall output, it should be easy to tweak one string or the other to get it exactly the way I like it.