Things have settled down now. The best I can figure is that the fish were freaked out by the noise (and possibly flow) of the new "quiet"one 6000 return pump. I swapped it out for a 3000 (much quieter) and they went back to normal behaviour. My poor bicolor must have had underlying issues...although he looked healthy and robust only a few days before
I haven't seen him for 2 days. The long term plan is now to get another 3000 and have twin return pumps. The 6000 can be sold to someone who doesn't mind enjoying their reef while wearing earmuffs...