3D is never easy .

Well at the moment it seems to me that you are using to many lines, and the lines looks to linear to me. (four sided polygons is not a must just a recomendation.)

And well it is okey to make advanced model outside the model and then comine and merge it, i usually do that on very detailed areas like the ear for example.

Add the mouth and we will see more how you do detailed modelling.

But dont worry, you will get the fish looking fishier in no time, good luck!

Ps: I might get the red pen around when I get home from work.