I don’t have fish in a tank and can barely keep a spider plant alive, but here is an article suggesting that if you do have fish in a tank, you need a school of your different kinds vs singles in order to keep them the happiest and stress free.

As Dr Katherine Sloman from the University of Plymouth explains ‘fish kept alone or in pairs show higher levels of aggression than those kept in groups of ten or more; large groups are also more likely to exhibit natural behaviours such as shoaling’.

At least for neon tetras and white cloud mountain minnows.
It makes sense to me.