I found this on another Forum,it is not my joke.

A woman brought a sick koi into the veterinary surgeon. As she
laid the koi on the table, the vet pulled out his stethoscope and
listened to the kois chest. After a moment or two, the vet shook his head
sadly and said, "I'm so sorry. It has passed away."

The distressed owner wailed, "Are you sure? I mean, you have not done
any testing on her or anything. She might just be in a coma or something?"

The vet rolled his eyes, shrugged, turned, and left the room,
returning a few moments later with a beautiful black Labrador. As the koi
owner looked on in amazement, the dog stood on his hind legs, put his front
paws on the examination table and sniffed the dead koi from top to bottom.
He then looked at the vet with sad eyes and shook his head. The vet patted
the dog and took it out but returned a few moments later with a cat! The cat
jumped up and also sniffed delicately at the ex-fish. The cat sat back,
shook its head, meowed, and ran out of the room.

The vet looked at the woman and said, "I'm sorry, but like I said,
your koi is most definitely 100% certifiably...dead."

He then turned to his computer terminal, hit a few keys, and produced
a bill, which he handed to the woman.

The fishes owner, still in shock, took the bill. "£150!" she cried.
£150 just to tell me my koi is dead?!"

The vet shrugged. "If you'd taken my word for it, the bill would only
have been £20, but...with the Lab Report and the Cat Scan......."