Sorry to drag up an old thread from the depths, however:
I have been looking into this recently and have found that no airline will allow you to check fish.
Southwest Airlines - No pets, period except for service dogs
American Airlines - Only cats and dogs, although they can refuse service based on temperment
JetBlue - dogs and cats in the cabin, no pets can be checked
Express Jet - Only domesticated cats, dogs and household birds, only in the cabin, crew can refuse pets at any time
US Airway - One small domestic dog, cat or bird per passenger is accepted in the passenger cabin for an $80 fee each way
Delta - Pets cannot be checked as baggage on Delta- and Aeromexico-operated flights. Dogs and cats can be taken in cabin, restricted to the list on the website
Thats just a few, Ive check all the large companies in the US and they all say about the same thing. Most airlines will ship pets as cargo but as far as I can tell the TSA requires that you become a "known shipper" to ship packages over 16 oz