Thanks for the comments guys.
The pelvic fins of my smaller occelaris are looking a little tattered. The orange sections are ok, but the black margins aren't looking too good. However there is no redness around the area, no white film, or any other signs of bacterial/fungal infection that I can see.
After watching closely for some time tonight, I did see the slightly larger one bang into it from below a couple of times, possibly taking a little nip at the fins in the process. They have both been a bit more animated tonight than usual, darting around more rapidly.
Both were bought at the same time from the same tank, from a group of around 20 or so juveniles. I've had them for about two and a half weeks, and deliberately picked one to be a little larger than the other, on the assumption that this would minimize any dominance struggles. They hang around together more than they're apart, and sleep laying side by side against each other on the substrate in the corner of the tank.
Is this likely to be just the result of one exerting it's dominance? I guess after the firefish issue I'm wary of fin damage, but in that case there was a white film and it progressed really quick (24 hours from nothing 'till dead) whereas this has been a gradual thing over a couple of weeks and there's no film.
I'll try and post a pic later, but for now I've got sick kids to attend to. And given the way they're currently darting around, I doubt I'd get a good shot at present anyway.