To clean bongs effectively, both soft and firm cleaning brushes can be used based on the specific needs of the device. Soft cleaning brushes, which are usually made of nylon or silicone, are gentle on glass surfaces and perfect for removing surface-level residue without causing any scratches or damage. Delicate parts of the bong, such as the percolator or intricate glasswork, can also be cleaned using soft brushes. On the other hand, firm cleaning brushes, made of metal or stiff bristles, are better for removing tougher, stuck-on residue and for cleaning hard-to-reach areas like the downstem. However, improper or excessive use of firm brushes may lead to scratches or damage to the bong. Hence, it is recommended to start with a soft brush and gradually switch to firmer brushes as required.