i kept mine for more than a year.
i thought doctor fish cannot tolerate another doctor in the same tank, maybe snatch business mine harass another to death
btw, doctor fish dont cure inch,
1) try getting a few cleaner shrimp, much better,
2) soak food in garlic guard
3) change partial water , good water parameter improve immune system , check ph also, low ph affect them too
4) or put in quarantine add copper.
hope your bt recover soon