I am somewhat new to the DSLR world so I may not be of much help with camera settings; however, I do have a little bit of an Art background so I may be able to contribute something: when I work with an image, I find the “negative space” (i.e. the space around the fish) is equally as important as the subject. The negative space can be used to set off your subject and draw your audience's eye where you want it. Try not to crop in too tight to your subject and look to include negative space in your composition. Another term that will aid you in capturing a creatively correct exposure is to compose your image with the “Rule of Thirds” in mind. If you are interested, I can try to dig up some info on the Rule of Thirds.
Beyond that, your fish is beautiful!
Regards,
Russ