Christian,
 
        Black tarry stools are an indication of stomach ulcers in the ferret,
and often are unaccompanied by other signs.  This may not be what the problem
is in your ferret, since it doesn't seem to be appearing consistently, but
you may want to have your vet check it out (don't worry -- treatment is easy
and effective -- peptobismal! -- but see your vet for dosage).  Also,
although even Dr. Kawasaki says anything but generic pet food brands are
okay, I personally believe that all pets, especially our sensitive carpet
sharks, should be fed high quality prescription diets like Iams or Science
Diet.  Yes, they are more expensive, but they are also more nutrient dense
and therefore more filling and disappear less quickly.  And I personally
believe they are far healthier for the GI tract and the body as a whole.
(I'm not a vet who makes money off of it yet, either -- I'm still a poor
student, so I'm saying this honestly! :) )
 
                                        Laura
 
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Laura L'Heureux, Trella, and Tribble, too
U. of Illinois Vet Med Class of 96
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[Posted in FML issue 0593]