That can be a sign of ammonia toxicity or nitrite toxicity... In general terms they are having trouble breathing.
Do you have CO2 on your tank? The others may just be too brain dead from oxygen deprivation to move to the top of the tank.
I would do the whole gauntlet of water tests before accepting this as normal behavior.
Do you have CO2 on your tank? The others may just be too brain dead from oxygen deprivation to move to the top of the tank.
I would do the whole gauntlet of water tests before accepting this as normal behavior.