creat a new file in photoshop as big as youd like the background to be.
click on the gradient tool (its where the paintbucket tool is. click and hold to bring up gradient.
top left is the colour. pick a light blue.
Next to colour (top left) theres a choice of gradient styles.(5 in total) pick linear.
Then on your new canvas, get the mouse and start at the top of the canvas, click and hold and pull down across the screen, then let go.
experiment for different effects etc.
Save as a PSD, TIFF or JPEG, depending on what your printers decide to use.
print in matt, and not gloss.
A word of wisdom, always get it printed bigger than the tank actually is, with wide angle lenses, the show more of the sides, so if your background isnt big enough, you'll see the wall.