Betta Fish: Female Betta Injury?

Please can someone help me! ls this an injury? Am I right to think the perameters are okay?
She's about a year old and living with 8 dwarf corydoras in a 80L. I top it up once a week. Other maintenance fortnightly.
Could the Corydoras have done this?

Extra info;
I've only been fishkeeping about 18months I'm heavy on research and even gotten to the point where I'm remineralising RO.
I think I ought to get a tds meter, but that's a different story, might explain the high GH… should I only be adding acid/alkaline buffer when only topping up not changing?

Thanks in advance 🙏