It's difficult to hazard a guess what went wrong with your fishes. "Roughed up skin" could mean various things. If the water chemistry goes wrong, many fish produce extra mucous on their skin, and that gives them a whitish look. It's possible that's what happened here. Definitely check the pH: if pH changes rapidly, it can kill otherwise healthy fish.
Ropefish should be fairly nitrite tolerant because they live in swamps and can breathe air, so while a filter problem could be to blame, it wouldn't be my first guess.
Is there anything in with the fish? I could imagine something like a plec rasping at a ropefish that was sick, and in doing so produce scratches or inflammation on the skin. I've seen Otocinclus attack large gobies and produce this sort of damage. Alternatively, could the fish have gone somewhere it shouldn't? Such as inside a filter or wedged behind a heater. I'd certainly put a heater guard around the heater if keeping this species (or for that matter any eel-like fish -- there is a risk of the fish wedging itself behind the heater when switched off, only to get burned when the heaters comes on).
Assuming your pH and water quality are good, then I suppose a disease could be the thing, but ropefish aren't particularly prone to getting sick. I would imagine their armour plating makes them fairly resistant to whitespot, though someone like beblondie would know better than I. Fungus and finrot are possible and both can cause white patches on the body, but these are rarely fatal, at least not in their early stages.
Cheers,
Neale
