If only the ASPCA cared about fish... In accordance with the animal welfare act, "animal" is defined as being warm blooded. While it may be cruel to fight fighting fish, it is not illegal on a federal level. However, it may be on a state by state basis.
I hate to say it, but Jaysee is correct. No one really cares about fish... If they did petstores would never be able to keep them in the conditions they do. If they kept a dog or cat or even a bird in a weeks worth of waste and a tiny cage, though, they would be charged. Sucks but that's reality. I wish it was illegal.
Here is the definition of an "animal" directly from the animal welfare act-
Under Section 2(g) of the AWA (7 U.S.C. 2132(g)), the term animal includes, with certain exceptions, any live or dead dog, cat, monkey (nonhuman primate mammal), guinea pig, hamster, rabbit, or such other warmblooded animal,