Ola Bryan
I do that sort of stuff for quite a while now. Being a Multimedia designer I have learned to use Photoshop proffesionally.
A couple of examples:
A reeeeeaaaallly bad one
One of the latest
And.... Something completely different:
Above is a modern pic made 60 years old. Result of an evening Photoshopping
In answer to your pic I have to say that you are on to a good start. Might I suggest you add some blur to the Tank and even a little more blur to the background. Not too much you still need to see the details only you have to get rid of the sharp lines in the pics (blurring can be found in the filter menu I would suggest Motion blur and set it between 1 and 3) Further you might want to add some contrast to the picture. Make the dark colours stronger and the light colours a bit lighter. Play a bit around with the Brightness/Contrast menu found in the Image/adjustment menu.
I see you have added filmgrain/noise very good touch. Overall very nice work on the pic. try my tips and see if you like the result.