A single schooling fish can become aggressive. If the species is meant to live in a school and they are alone, out of fear they will instinctively try to protect themselves. Keep at least 3 of a species that school together.
Re-arrange all the ornaments in the aquarium each time you do a water change. Over time, a fish will be more and more protective of its territory, but re-arranging literally resets everyone’s territory making them start over and be less aggressive towards others.
Keep semi-aggressive fish together. The fish will be able to tolerate being chased since he typically behaves this way himself. With this set up, everyone will be chased and will be the chaser, so a single fish will deal with less stress since it’s all evened out through all fish.
Make sure all fish are the same size. Smaller guys are easy targets and tend to get bullied by the bigger guys.