Gale Pearce wrote:
I would check them closely for fin rot - do any of their other fins appear
to be getting shorter or stubbier?

In addition to fin rot, the tails might also be getting nipped. There's
also the possibility that this is just a perception issue. Never discount
the ability of the mind to distort perceptions to fit preconceived
notions. It's the reason several optical illusions work. Without absolute
measurement numbers, there's no real way to tell for sure.