missed this post - I am so sorry
I breed L071 peppermints in my main tank along with ancistrus teminckii, my peppermints used to be in a 4ft tank on their own, but will I am in the process of moving home they are sharing a 80cm fluval tank (not the best option), and can safely say the peppermints can be aggressive. Far more aggressive than other species of bristlenose catfish.
While the majority of time they will be a peaceful species :P
Aggression in peppermint catfish can happen for a number of reasons.
A female peppermint can become overly aggressive around her male at mating times, both before and after mating,
infact to such a extent I once saw a small female peppermint having a mad attack on a male nearly twice her size.....
Peppermints can be aggressive and territorial around their caves, aquarium plants, food, and favourite resting spots.
Observe closely and if it happens again then a short term isolating in another tank with a few other fish can often help.
As others have stated feeding at different times and also breaking food up into smaller bits can help, as can feeding the fish at night before bed.
Sometimes if the peppermint which is showing signs of aggression can be isolated in a smaller tank by the side of the main tank this can help-
eg: so she can get accustomed to the site of the other fish, and not start a fight, and they can have lower aggression this way.
If the tank is big enough you could split the tank with a divider, though I would suggest this being one of the last options.
Sometimes you just get the occasional peppermint's that turn aggressive for no reason to each other, as you can with other fish,
and if you can not deal with it then it may be that you have to pass the peppermint on to another breeder or fish keeper ...
I know this will not be what you want to hear but sometime's it can be the only option left, and be the only way of helping that
fish survive, or alternatively protect the other fish in the aquarium.
It is testament to your good fish keeping that the peppermint is now improving.
I do hope all goes well for you and your fish.