Hey,
I'm new here.
Anyways, here are my thoughts about the fish.
The coelacanth is often used to symbolise something which was presumely dead, but is actualy alive.
It's also used to symbolise Human beeing since the fins of the coelacanth are early feet. That specific fish marks the step from water life to terrestrian life.
Now, I still don't know what that implies about Blame! since I've only read up to the 7th volume and I didn't analyse it yet.
The manta ray must have it's own meaning too like all the surreal parts of the story.
In that vein, I noticed something that I haven't analysed in depth yet. The caterpillars (at first I thought they were worms) in the begining of the story are symbolising the whole quest of Killy and Cibo. We see them eating the megastructure, destroying it, but it only leads them to fall down and crush a few levels below (and the violence of the scene is important). Later, we see Cibo flying with butterfly (evolution of caterpillar) wings. It seems like her path to freedom.
Well I'm just throwing some thougths. I'll pass by again when I've read the whole volumes.
Later.
-IS-

One problem though, throughout the entire series, cibo's personality is always transferred through the net, not magic. The fish is (most likely) not cybernetic, so how could she be contained within it.

Anyways, for some reason, the coelacanth appears in several animes for unknown reasons. It appears in Tezukas Metropolis among others. Maybe it is symbolic of something that has lived beyond it's time, that's all I can think of.

If I could type Greek, I'd put it up. Sadly, I have no clue how to do it. My guess is that it's probably gibberish though. Looks like it's supposed to resemble computer programming.

Here is something interesting i found when looking through the artbook again: the fish is swimming in an electronic cyberspace sea....