Cleaner Shrimps are omnivorous scavengers, eating almost anything, but are most-well known for their symbiotic relationship with many fish. When a fish comes down with skin infection, it will sidle up next to the shrimp and patiently tread water while waiting for the shrimp to clean its body of parasites. For the shrimp's part, it is usually quite diligent and will pick dead skin and necrotic material off the fish, keeping it disease free.
