I think we have identified the main culprits in this thread.
But I want to add something on the flow and agitation.

If flow is very strong or too strong I see fish from guppies to tetras take refuge in quieter parts of the tank where the currents are cancelled out and are not too strong. It strikes me that a powerhead and bubbler and filter is a lot? If all of those are going i would not be surprised that the fish maybe sticking to quieter parts of the tank.

If the powerhead is aimed upwards to the surface its still going be shifting X gallons per hour even if the output is not as concentrated. I suspect that along with schools of limited sizes that the currents in the tank might be too much. Hillstream loaches will probably love it and want more and guppies probably won't care one way or another. Where are these tetras from do they inhabit turbulent fast flowing streams or do they inhabit larger bodies that flow more smoothly.

Just a thought.