Hi Magma - MoI is a CAD program and overall it's more oriented towards making mechanical type shapes and forms.

An actual fish shape (rather than just a straight-sided punched out 2D outline of a fish) is an organic and non mechanical shape.

MoI is just not really a very suitable tool for doing things like that.

It's possible to do semi rounded profiles by making a rounded capping surface and cutting with that, see some examples in these previous discussion threads:
http://moi3d.com/forum/index.php?webtag=MOI&msg=606.4
http://moi3d.com/forum/index.php?webtag=MOI&msg=3137.5
http://moi3d.com/forum/index.php?webtag=MOI&msg=2909.2
http://moi3d.com/forum/index.php?webtag=MOI&msg=2666.5
http://moi3d.com/forum/index.php?webtag=MOI&msg=1782.4
http://moi3d.com/forum/index.php?webtag=MOI&msg=4296.5

But if you really want something that's very organic fish shaped and not some kind of stylized shape that's more suited for CAD you will probably need to use a polygon mesh or sculpting type program rather than a mechanically oriented CAD program such as MoI. Maybe check out Sculptris or Cheetah3D...

- Michael