Yep, was thinking about doing a water change and putting some salt in the water, but thought I'd see if there were other ideas first...
It's not a peat substrate, at the moment they are in a tank with a sand/small cobble substrate, with some larger rocks for hiding under. There's also freshwater mussels in with them. I'm pretty sure their fins were fine when I first got them; have had them for almost two months now, with the fins only starting to show damage in the last few days. Not sure what the comparitive pH levels are though; also, not my tank, so not sure about the filters...