After watching the tank past couple of nights I've re homed 7 blue tetras today. Seemed to be the main starters of the aggression. Similar to
@zozo story of the other fish didn't dare get in their path. Only the first evening lights on and you can see the difference already, everyone has free passage to swim freely.
If you read the description
http://www.seriouslyfish.com/species/knodus-borki/ it's a valid candidate causing agression and stress in the tank.. With choosing such characters you always need not only to look at characters of the other species also the region they like to go around, like top level, mid level or bottom level of the tank.
In terms of numbers you might think your tank is not overstocked, but if all live in the same level it still might be the case and fish can feel cramped up and will result in agressive behaivor towards eachother.
They do not nip fins for the fun of it, it is a form of territorial comunication and a sign of stress, ussualy for fish which are cramped up and are in eachothers space to much.
In time if you do your homework and proper observations you'll learn, not only to choose fish which can go together but also how setup a tank for them, with choosing the right planst and hardscape suitably for the size, shape needs and behaivor. Create hide outs, hang outs, shelter, break lines of sight etc.
For example if you have a fish sp which like to live in the upper regions of the tank, then give it some surface vegitation, this doesn't absolutely need to be something floating but it also can be a large java fern attached to hardware close to the surface which creates a overhang with it's long leaves kinda floating. Then you will notice that fish loving the surface hang around there becuase iot makes them feel safe. That's what they do in nature, under floating vegitation or close to the shore under overhang from marginal plants.
Imagine as a bit overdone example.. If you were hardcoded by nature to feel safe close to shelter, because you are low at the food chain. Would you feel safe if you were put on a mountain meadow with no trees and shrubs in sight for miles?