What’s the secret ?
Constant feeding throughout the day ?
I'm not sure which of these had the most (least?) effect, but this is what I've done in my reef:
• The reefscape is all low-profile (1/3 height max), so at least 2/3s of the tank area is just water to swim in.
• There are lots of crevices, caves, nooks and ledges for the chromis to hide in and sleep at night. I literally find new ones that the fish have explored and turned into sleeping spots every month.
• I feed 6-8x pellet feedings (automated) and 2x frozen feedings daily; the herbivores get 4x separate seaweed feedings to keep them occupied.
I have 5x blue-green chromis, 4x lyretail chromis, 2x bicolor chromis and 1x black bar chromis. The blue-green are "free swimmers" while the rest really seem to enjoy exploring in groups. I don't think I could keep a large group of just blue-green chromis without occasionally losing one every month or so (just seems to be the way it is). I had 6x blue-green chromis for the longest time but one got pushed out of the "swarm" and I found him on the floor one morning (first time I've lost one to jumping out of the tank).
I also have 2x lyretail and 2x marcias anthias which co-mingle with the chromis (similar behaviour). I have 3x blue sapphire/devil damels and 1x azure damsel which have divided the tank up into quadrants and roughly act as "reef shepherds".
I have good flow in my tank but not obscene flow (3-4x tank turnover with the return and 10x with my powerheads) - so not that they would struggle, but it's more of a gentle current than a torrent.