if you have photoshop:
open both the background and 3d fish in seperate files
copy the fish over to the background file so it looks like what you have above
then do some colour correcting:
in your layer control, select the fish layer, at the bottom of that panel theres an icon called "adjustment layer" click that and use the colour balance to adjust colour, then hue/saturation to adjust the saturation of the layer
its all about playing about with different adjustments til you get it perfect
theres no right way to do this, you could even do it in combustion/after effects, or even maya

let us know how it turns out