Pentazona Barbs Dying - Old Age?

Hi there, I am new to this site today but have had a 180l tank for a year and 9 months now. Up until now it has been a very healthy tank and I have lost very few fish. However, this week I have lost two of my pentazona barbs and another couple don't look great  . I have quite a lot of fish in the tank but do water changes every 10/11 days and am careful not to over-feed. All of my other fish look healthy (plec, cardinal tetras, penguin tetras, Odessa barbs, red serpae? tetras)

A couple of months ago I introduced the Odessa barbs (3 male, 3 female) and recently I have noticed that one or two of the males can be a bit boisterous. I have never seen them nip any of the other fish but I have noticed that my pentazonas are missing bits of their dorsal fins. The fins of all the other fish are fine. The behaviour of the pentazona shoal hasn't changed and there is no sign of any disease.

My thoughts are that they are dying due to either stress or old-age  . Does anyone know of a disease that affects these fish in particular whilst leaving the rest of them untouched? Any advice would be appreciated as I have had my pentazonas from the start and I am very fond of them!