OK, i will explain how it's in my tanks.
The overflow from my display tank goes through a hole i drilled in the back of the tank about an inch below the top line. A pipe knee connected inside the tank to this hole and ensure that the water line will stay 1/4" below top line. On the back side there is a pipe going down to the bottom tank (2.5 gallon, the display tank is 10 gallon...) and release the water on bag full of fiber filter and activated carbon.
The sump is partitioned with a piece of acrylic with holes to release the water from one side to another.
In the side of the water fall there is crashed rocks with macro alage (Chaeto). In the other side there is the pump alone (for now, i need to add protein skimmer). The pump sending the water back to the display tank through a pipe. This pipe goes over the top, not through a hole (i didn't do a second hole), to the tank and flows the water inside.
If i won't use the check valve, when i am closing the pump it will backflow to the sump and overflow it. If i am using the check valve, the flow to the display tank is very low.
Is there a way to prevent backflow without harming the flow rate?