Fish Behaviour And One Fish Becoming A Recluse

There seems to be a definite pattern to the fish's daytime activity:
* Mornings consist of the frantic chasing / mating behaviour by the tetras
* Early-mid afternoons consist of the fish finding a staging post and attempting to rest but with frequent re-ordering due to a fish (harlequin or tetra) pushing and chasing another away, involved in a duel with another fish (usually the two smallest tetras), or, if it lays claim to one of the two open areas of the tank, chasing any fish away which enters its territory; mid-late afternoons consist of friendly swimming
* Evenings consist of rest-time

I've noticed that one of the two smaller harlequins has taken to spending most of the day over the past few days nestled in a leaf high up in the tank, not even moving when I open the tank lid to feed the fish. I'm guessing that it's choosing to spend its time there either due to being chased up there, to stay out of the way of the boisterous activity below, or possibly due to being weaker as a consequence of lack of food (as the two smaller harlequins tend to wait until others have eaten and often drop any food they do catch, presumably out of surprise/excitement at actually getting any). All fish look perfectly healthy and well-coloured, so, at the moment, I'm not too concerned although will continue to keep a very close eye on it. Any other suggestions re what I can do? I did contemplate buying one additional harlequin (about all stocking would allow) so that there would be another small harlequin for company and to even out the size distribution but not sure that would necessarily help at all or that a harlequin on its own for a fortnight in a QT would be very happy.