Depends on the type of fish and how beefy the filtration is. Cichlid tanks, for example, have to be heavily overstocked with big beefy filtration (including a sump that expands the water volume substantially) because if you stock at normal amounts they kill each other. And cichlids are very common.

Might be tetras, which prefer large groups.

And they prob don't enjoy that light shining on them that intensely