For a small tank, posterboard would be easiest. I would not bother laminating, it would be cheaper just to replace it once in a while as it becomes waterstained. Black vinyl, perhaps cut from a shower curtain, could be a good waterproof idea.
If you are not planning on changing colors, black paint would give the best results. Posted via Mobile Device
I just used spray paint. I put the fishies in cups and tore the tanks completely down, took the tank outside and sprayed the back. I taped plastic to the sides so it didn't get on any other part of the tank.
I have a 2.5 gallon so I took a piece of photo paper and printed out a picture and taped it to the back of my tank! It's been in tact for a year now and water hasn't ruined it yet! How big is your tank?
I have a roll of childrens tan coloring paper. I just cut it to fit(fits 55s perfectly) and painted the paper black with acrylic paint. It has gotten wet some a few time, but is totally fine. Unless you are making huge messes with water, paper should be fine. You can also use either electrical tape or painting tape and easily take the background off if need be.
I would revisit those "Ocean Backgrounds" that you are so against.
The back of them is ussually either Dark Blue, or black.
Myself, I was originally against the idea of those "Aquascape" backgrounds, very much so. My LFS had a Black on one-side, Blue Gradient on the other that I liked alot. I purchased it for $2.50 for my 10 gallon.
Exactly what I wanted, but now that I've had it up for a while, I really wish I got one of those with an artificial aquascape behind it.
I currently am using the blue side. It took a little effort to fit it properly (trimming, coating with oil, rubbing out all the bubbles with a credit card).