It's difficult to create true laminar flow in an aquarium. The shape of the tank tends to destroy the layers and we end up with turbulent flow instead. Of course I'm not pHD in hydrodynamics but that's my understanding. Which means that generally there is a transmission of flow power. We don't see this once we get below the surface because there's nothing to indicate it to the naked eye very well. Of course I'm not saying with any certainty that this is an issue in your tank but fish hiding behind something that can block flow makes me wonder.
My dad was a mechanic, in the old days, before computers could tell a person what was wrong with a car. He taught me to diagnose the engine by process of elimination starting with the easiest thing to eleminate. If a car won't start check the battery, if that's fine, check the ignition, etc.
Knowing in general what might cause an issue I do the same with fish tanks. This is helpful when the issue isn't readily visible. Ich, fungus, wounds, are easy to diagnose because they can be seen, but internal parasites, old age, etc. aren't so easy. I try to eleminate what it might be one step at a time and in general I've found that this works. Diverting flow, may not be enough, it may require an actual reduction in the speed.
