Thanks, DarylMac! And I've considered the construction paper route as well but wasn't sure how it would look if it were to get wet... so I actually took a different route (believe it or not, cheaper too!)The fish and tank are looking great! The cheapest black background I have found is just construction paper from the dollar store - it's $1 for a 24" x 28" piece. What I do is tuck it under the top trim of the aquarium and tape in place (the tape is not visible from the front of the tank since it's on the backside of the paper and the trim - you're effectively taping the paper to the trim, but the paper is tucked under the trim so no light gets through).
Then I use a box cutter to slice the paper at the top of the trim on the bottom. I don't tuck it in because I keep my substrate at a depth higher than the trim so any gaps are not visible. voila! That's it.
It takes about 2 minutes, and costs next to nothing, you can do it with water in the tank (if you can reach behind the tank).
I'm not going to say it looks as nice as a painted black background, but it looks good and costs a lot less and takes a fraction of the time.
I had a box of black trash bags, I just taped one to the top of the back of the tank and let it drape down the back... since it was slightly wrinkled it carried the slight illusion of depth from certain angles now I just paint it black lol I have a quarter gallon at home and plenty of paint supplies from when I painted in my summers between years at school