Here is the most efficient way to cycle a LOT of sponges:
1) 3/4" PVC on the bottom of the tank running the full length. 90 degree fitting at one end, tee at the other.
2) Power head at one end fitted to the PVC 90, or to a short piece of pipe in the 90.
3) The tee simply sits at the bottom at the other end so water can enter, and holds the structure stable. A bit of pipe in each part of the tee can help, but not long enough to end at the front or back of the tank.
4) Pack the aquarium full of sponges except right where the power head is. Allow a gap so the force of the water is not deflected to splash out of the tank. Keep the power head high enough to create some ripple in the water at this point (better oxygen level in the water)
The intake of the power head is via the PVC pipe at the other end of the tank.
The water has to flow through pretty much all the sponges to get there.
For a bit more about this idea have a look at this:
Never mind reading through it. Scroll to the bottom and look at the manifold. You are not even building one that complex. (though you could).