[tang daddy]

black thing on the fishes tail is alittle bit weird, never seen that before it could be velvet....

if the fish is thrashing around, or as I like to call it skipping off rocks then there maybe ich. Just see if there are white spots on it, the spots are small.

Ich is in every fish, they carry it and when stressed they have an outbreak.
feed it garlic soaked food and do weekly waterchanges of atleast 30%.

If the black stuff on it's body fuzzes or gets wierd like an open wound I would qt the fish in another tank to stop it from spreading to your other tank mates.

When you buy fish in the future make sure you fully inspect it for fungus and parasites.

Personally when I buy fish I like to sit and look at it for atleast 10-20mins, the reason I do this is to study the actions of the fish. Believe it or not you can tell alot from a fish by how it swims, sometimes they have bladder problems which cause them to swim funny also a fish that is not active most times has problems. And a last thing please do research on the fish before buying it.... IE tank requirements, food, compatibility with tank mates you already have, these play huge rolls in the survival of the fish!

I am not an expert but have read the marine fish book over 100 times so I know exactly what the requirements for every fish are.

GL