Have used Hydro sponge filter's for a few years now.
Airstone should be inside the sponge and depending on the air pump, the force of bubbles rising in the tube, will provide some mechanical filtration but mostly biological.You will be surprised at the dirt,mulm,that is squeezed from these filter's with weekly cleaning.(squeezing)
The longer the hose from the pump to the sponge filter,,the less the force of bubbles will be rising in the tube. Shorten the distance and bubbles will increase.
I use luft air pumps which are plenty stong enough to run two Hydro V sponge filters in a 75 gallon tank. They are a bit pricey, but last much longer than my second choice, Whisper pump rated for 60 or 70 gal I believe.
It is the force of the bubbles rising in the tube that pulls dirt,mulm, into the sponge. Weaker pumps,or too long of hose,will decrease the force of bubbles rising and thus not much will be drawn to the sponge.
Hope this helps.