Sponge Filter Troubleshooting Picture

SPONGE FILTER POOR FLOW TROUBLESHOOTING

Although mechanical filtration is not the main strength of a sponge filter, a Sponge Filter can still can remove copious amounts of debris form the water column when properly connected.
If not connected properly, you will see little if any mechanical filtration and even biological filtration will suffer as well.
This can be a problem with air pump applications, especially in small aquariums where the aquarium keeper unknowingly leaves off the lift tube to fit better in a small aquarium and then amplifies the problem by not extending the airline deeper into the sponge filter

The picture above shows water flow through a sponge filter depending upon air diffuser, airline tubing, and lift tube placement (Please click to enlarge).

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As you can see a sponge filter on the left with no lift tube and with no extension of tubing (or air diffuser) has a poor flow through the sponge material (if any flow at all).
The reason is the air bubbles rising in column will create a vacuum, however if there no lift tube (or very little) no vacuum is produced and the water will simply flow upward with the current of water, mostly avoiding the sponge (which will have more resistance).

The point is to overcome the resistance of the sponge material with a stronger vacuum produced in the lift tube, as well a an extension of airline tubing into the sponge filter and/or the use of an air diffuser or air stone can extend this vacuum deeper into the sponge filter, thus providing a better flow.

FURTHER READING:
*Aquarium Sponge Filter Use/Information
*Freshwater Aquarium Care/Information