Your pump is outruning your flow from the tank..either bigger line/fittings from tank, or put a way to control your pumps output. I have used ball valves, didn`t like them(to hard to get just the right setting I wanted)gate valve is the choice, little more $ but worth it to me for the control I got. "T" out of your return line, main back to your tank, the leg of the "T" with the valve would go back into the sump..that way you control the water level in the sump. I would also look into a float switch to protect your pump, should something stop the flow of water to the sump..A few $ for a switch could save the most expensive part of your system.
I never used Bio Balls, pot scrubbies, and none of them were submerged. The water flow thru them was the whole idea..wet-dry..wet=water flowing thru them..dry=air from not being submerged.
Bacterial growth is enhanced by the amount of air available by not being submerged.
HTH