One last followup to my 'experiment' of running black volcanic sand despite initial significantly elevated vanadium and nickel levels.
On the most recent ATI/Triton tests of 1/10/19, nickel and vanadium have further dropped and are no longer elevated, now reading 2.1/0 ppb and 2.6/0 ppb, respectively.
So, it appears that the leaching of these metals from the black sand has ceased? Or at least dropped to a very low level such that water changes control it completely. This is just under 11 months after tank startup, and I do 10% water change weekly.
It's hard to see why this would be the case. I would guess that the sand still has lots of both metals within it. Maybe metal ions can't migrate from deeper in the rock so only surfaces leach. Perhaps the individual grains get coated with a biofilm that prevents further leaching. I do siphon the sand now and then to remove detritus, so there is some turnover. I probably won't get a definitive answer but I'm happy with the result.
I will also say that overall corals and nems in this tank are doing better than ever now, and that there were a few unexplained deaths earlier. I can't say for certain though whether the excess heavy metal levels were responsible.