Perhaps if I put some bioballs instead the
stones, it will work a little better since there would be some more
turbulence.

That's just my point, the 301 didn't provide enough turbulence. I have an
empty gravel vac, suction cup holding the hose in teh middle of the vac
tube, a 2" sponge piece in the bottom and the Magnum providing water flow.
The turbulence is so good I don't need anything in the tube to create more.