Here, the plastic aquarium Backgrounds are about $4/ft or something completely ridiculous like that.
Can of spray paint: $1.50
Roll of black or blue Wrapping paper: $2.50
And my personal favorite, 3M adhesive drawer-liner, 25' roll: $6
if you plan on turning it into SW, i heard that salt creep can get behind the paint. btw, i painted my 150g (48"x24"x30") with a quart of paint. 7 coats and i still have a half can left
I spray painted a 1" thick piece of styrofoam and used a few dabs of silicone in the corners to attach it to the back of my 55 since it sits against an outside wall and the temp fluctuates greatly here. It insulates the aquarium nicely and has looked great for two years. It did take several coats to get it nice and evenly black. You also need to cut out a couple of areas to fit around the HOB filters if you have them.
About a month ago I paited the back and one side of my 220. I used a quart of Behr hi-gloss pure black from home depot. I love it. I was looking at it yesterday and saying to myself how good it looked. On resale nobody will be mad at the black paint
dark window tint, either from an auto parts store or a home supply store. Slightly more money that some other options but the look is equal to or better than that of painting the back, but easier to apply and remove