Delta Cargo has an exception to having an account if its a live tropical fish. You need to call there cargo booking people and specify that its a live tropical fish, if they say you need an account ask to speak to there supervisor, they don't all know what they are talking about there. Call 800-352-2746 and hit option 6 for there live animals dept, best chance of finding someone who knows what they are doing.
I have an account with Southwest Cargo, there signup process was rather silly, but not difficult. You just have to call them up and tell them you want an account, the will have a third party contact you to send someone out for a "site inspection". A guy came out to my house, stood in my living room, made a photocopy of my drivers license and a utility bill, and then left, that was the site inspection. You can sign up with a personal account with them.