I have to YBS in a big tank (50x50x150cm). Recently they have been spending a lot of time sitting in a coupke of flower pots inside the tank. They only started doing this about a week ago and it concerns me slightly. Is the water too cold (24 degrees C) and they are trying to hibernate? Are they scared of something?
Maybe he is the loser in a dominance fight or is weak from not getting enough food. You can raise the water temp to 79 or 80 degrees and see how that works out.