What is my harlequin rasbora doing?

I have a shoal of 7 harlequin rasboras in my 55 gallon tank with a few other small cyprinids. One of them has split off from the shoal and is hanging vertically, head up, in the water column in the corner of the tank, near the sillicone sealant. Every once and a while it will pull away, and snap back into normal behavior, then after a minute or so return to hanging vertically in the same spot. Sometimes another rasbora chills next to it, but in a normal swimming style. It's the same two, and they seem otherwise healthy. They are active except when in the corner, and both ate when fed. They are each about 1.5in long. I siphon out about 15 gallons of water and replace it every Wednesday. If it helps, I added a clump of java moss, three valisneras, and five gold barbs today, though it was occuring before I added the new items. Is it alright?