List of aquarium diseases

Amylodinium (marine velvet)
Anchor worms
Columnaris
Cryptocaryon (marine ick)
Dropsy
Fin rot
Skin or Gill Flukes
Ichthyophthirius (white spot or ick)
Velvet Disease, including Oodinium
Tuberculosis
Hexamita (hole in the head)
Lymphocystis
Flexibacter columnaris

http://en.wikipedia.org/wiki/List_of_aquarium_diseases