Conveniently, these fish prefer affected skin, and will leave healthy skin alone. The fish can be of two different species: Garra rufa and Cyprinion macrostomus. Many scientists think that as the fish remove your dead, scaly skin, light and water can get to the healthy skin underneath, helping your body heal itself.
It sounds like a joke, but according to one study published by Oxford University Press, "Ichthyotherapy (therapy with the so-called ‘Doctorfish of Kangal’, Garra rufa) has been shown to be effective in patients with psoriasis in the Kangal hot springs in Turkey."
In Kangal, the water's high temperature makes it difficult for any nutrients to survive; the doctor fish are therefore ravenous. Handily, they also have a penchant for dead, diseased or scabby skin. With their gummy mouths they strike and lick the psoriatic plaques, eating away the scaly skin that has been softened by the warm spa pool. Their nibbles can cause minor bleeding, which the selenium-rich water and high-altitude Turkish sunlight then heals. (source)
Click here to watch a BBC news report on a fish doctor spa.