Mine do it.
It reminds me of little girls playing advanced types of jumprope- when you're trying to zone into the proper rhythm to jump in on? You just sway a few times to get it right?
My guys all hang upside down in a row under a rock overhang when they do it. I thought maybe the current was too strong but I don't even have the closed loop running yet
mine sway to I've noticed that their moutparts tend to move overtime when doing so. I wonder if they could be trying to filter feed? From what I've seen they do it more when I've just added food to my tank.
chris