You can find National Geographic Magazine on Internet. In checking I found that their website says, "Generally, the full text of our magazine articles is not available online." So, a quote from the printed Nov. 2001 article:

Like a jowly king at ease with his attendants, a hippo passes with a retinue of fish. It has long been known that hippos and fish are constant symbiotic companions. Fish clean hippos and are in turn nourished by the algae, parasites, and dead skin scraped from the hippos' hides. What wasn't known ... amazed us. We discovered that certain fish specialize in cleaning specific body parts. The ubiquitous Labeo, in the carp family, is the main cleaner, using its wide rasping mouth to scour a hippo's hide. Barbus feeds directly on dung and cleans the cracks in the soles of the feet. Small cichlids graze around the tail bristles. And tiny Garra cleans out wounds.

Hippos are far from passive recipients of these services. We saw them deliberately splay their toes and spread their legs to provide easy access or to solicit cleanings. They would even visit "cleaning stations" where fish congregate -- much like pampered clients going for a massage or manicure at a spa.