Fish Fighting / Suggested Decor Alterations

After hoping that the tank might be problem-free for the foreseeable future as it was well balanced with fish of similar size and temperament and behaviour, a fight broke out this morning.  There are occasional daily squabbles but this episode started suddenly, became quite violent and was prolonged - I was working from home and could see them beyond the laptop.  Two tetras were swimming in conjoined fashion, covering the entire length and depth of the tank at some speed, then a third joined in.  There were “spats” among the silk plants, behind the rock and out in the open.  The remaining fish then got involved in their own similar mini battles/chases – the remaining 3 tetras with one another, the harlequins in twos, and anyone (tetra or harlequin) who had the misfortune to end up near the original 3 tetras got driven as far away as possible.  At some stage during this episode, one fish incurred an injured caudal peduncle with a protruding fleck very similar to the recurring one that one of the cories had.

I ‘m assuming that this is all about who is claiming the newly available territory previously occupied by the cories and/or re-establishment of the pecking order as it’s very similar to the behaviour when I first got the tetras and the harlequins.  Water readings are all good, and fish's colours are all very vivid as usual.  I don’t suppose that there is much that I can do about preventing future fights like this although I wondered what thoughts were on the following:
•   Indian almond leaves, said to have mildly sedative/calming properties
•   Replacing the potentially injury-inducing rock cave with a glass jar cave, an idea I got from an aquatic shop – as fish don’t see the glass of their tank, might this cause potential collisions, especially as decor gets inevitably slightly misplaced during/after weekly tank cleaning?

That's very helpful, thanks, and a good suggestion re how to adapt the glass jar idea.

They're x-ray tetras which I've now had for 11 months. The two largest ones (the greediest two and possibly the two females on account of their shape) have always had quite strong personalities, hating the tanklights being on and cowering (to the extent that I stopped using it as they wouldn't adapt to it but they do have a table sidelight on for some of the day when I'm at home) and hating water changes (although they're much better now since the harlequins joined them 4 months ago as they move around en masse during water changes rather than spending up to an hour cowering and hiding afterwards), and swimming up quite menacingly immediately behind the cories when they foraged in order to get any titbits of food that they might unearth. This morning's fight started off with one large one and one of the smaller/thinner ones, then another smaller/thinner one joined in - so perhaps it is possible that they were pairing off and another male decided to compete for the female. I'll keep my usual close eye on the situation, though - but at least I know the "fleck" was caused by an injury rather than anything else.

Thanks, Diz1.  The injured fish now has a raw-looking wound in place of the "fleck", visible from both sides (perhaps because of the x-ray body), but otherwise is behaving normally and with vivid colours; will just have to keep a close eye on him and the tank situation, though!  Thankfully, it doesn't look as though any other fish have been injured today while I've been away.

Thanks, Alex. I've got driftwood in the middle and plants dotted around for that purpose, and each fish seems to have created its own territory between these, although I've noticed one of the large/female tetras taking ownership of one half of the tank, sending everyone else into the other half, particular at/after feeding times. I might experiment with your suggestion (or some sort of variation of it) during tomorrow's water change.