Super Clean water is the key. My Freddy had a similar gash on her about 3 weeks ago. Nobody else was worse for wear or scratched so best i could figure she nailed the edge of the internal overflow and tore herself up pretty good. I upped the water changes dosed with a bit of salt and she's has healed up nicely.
Keep the water clean and stress down and I bet she'll be ok.....sucks you're so far away though