What I found interesting about the story was that Fishhead was caught in a rift between two different societies; the society of man (that largely rejected him) and the society of the other/the catfish (that apparently welcomed him with open fins). I don't know that this is so much a story about his proto-Aquaman abilities to commune with water creatures, so much as a story about social interaction and acceptance, as the two acts of violence are enacted by the two societies that Fishhead is in contact with.
First, you have the two brothers, members of the human race which, despite any deformities, Fishhead is also a member of. Unfortunately, Fishhead is different enough that he can be cast in this role of the other, allowing the act of violence to be more easily perpetrated.
Second, you have the catfish, a race that, although they have no connection to Fishhead according to species, seem to have become a kind of second family to him, and as such, seek vengeance upon his death.
On the one hand, this dehumanizes Fishhead even further, in that the only society he can find comfort with is in non-mammalian beasts (which are typically alien to mammalian bonds), but it also dehumanizes human society as a whole, because it takes a creature from the muddy depths of a river to accomplish a level of social acceptance that the world still hasn't quite reached.
The catfish do not see deformity or race, they see a member of a society. Unlike the brothers, who are urged to kill for the fact that they are slighted, the catfish only take a life after one of their own are taken. It is a bloody affair, and not ideal, but the catfish abide by the social contract of mankind, until it is shattered. The creepy thing being, that this is also a very human response.