Ichthyotherapy is the use of fresh water or marine organisms (mostly Garra rufa) as agents of skin wound/condition cleansing.
This practice promotes itself as a treatment that alleviates symptoms of psoriasis.

One study (and that is the only article I could find about the subject) states in its abstract:

Yet, the article about the Doctor fish on Wikipedia says the following:
So, is ichthyotherapy pure quackery or is it a medical treatment in its infancy?

Sources:
​Ichthyotherapy – Wikipedia, the free encyclopedia
Grassberger, H., Hoch, W. (2006), “Ichthyotherapy as Alternative Treatment for Patients with Psoriasis: A Pilot Study”, Evidence-Based Complementary and Alternative Medicine 3(4), pp. 483-488
Doctor fish – Wikipedia, the free encyclopedia