For a piranha, that’s barely injured.
Piranhas by nature are fin eaters, and that includes with each other. Since piranhas pick off weak or injured ones, so piranhas must heal or adapt fast.
In a week or two it’s fins will be back to normal. No need to help, piranhas are known to react poorly to medicines and have excellent healing rates and immune systems. All you have to do is keep an eye on it to make sure it isn’t being bullied.
They do fine with damaged fins as well, a silver dollar of mine is missing a pectoral fin and acts completely normal.
Characins as a whole heal fast, I had a banded leporinus (headstander, relative to piranha) get torn up (courtesy oto look like this:
In weeks you wouldn’t know it was injured besides him missing an eye.
