Cheap 3D styrofoam background
This cost me about 120 bucks. 70 for the tank and stand. The rest was in misc supplies. The most expensive thing was the filter which I bout new for 115.
Also my first instruct able so hopefully I didn't leave anything out.
Enjoy

Step 1: Gather Materials

Collect styrofoam. I got pieces from shipping boxes from work. You could probably also find them in a garbage bin somewhere. Make sure they are fairly clean. Once you start you'll notice that there are different types of styrofoam and each type cuts differently.

Any size aquarium. I bought a 55 gallon off craigslist with a stand for 70 bucks. The top brace was broken but easily replaced. A new brace cost me 20 bucks
Paint
Paint brush
GE silicone 1. This is important. You could also use aquarium silicone but it's very expensive. The GE 1 is the same thing and runs about 4 bucks a tube. For this tank I used about 8 tubes with plenty left over.
Latex gloves
Knife

Step 2: Cut Styrofoam to Desired Shape

I wanted my tank to be as natural looking as possible. I cut my styrofoam into long semi flat rock shapes. I also cut more pieces to look like small round stones. Any shape is possible. If you need a larger whole piece you can glue two together using the silicone. Just let it sit and dry for 24 hours. And paint. I used non toxic paint. I've also heard of others using krylon fusion

Step 3: Glue Pieces to Tank

After I painted the faux rock pieces I sealed them with silicone, the entire piece. The back of the long rocks were flat so I put a bead of silicone on that part and put it onto the back inside wall of the tank. Some of the pieces slide so I used another piece of styrofoam and wedged it against the front of the tank to hold the rock in place til it dried. They will dry firm in 24 hours.

Use gloves when dealing with silicone as repeat exposure can cause chemical burns that will look like a nasty blister. I learned that one the hard way.

Step 4: Add Plants

Add fake plants to your pieces by making a small slit in the already sealed piece and placing the stem into the slit. Then seal the slit with more silicone. I used plastic plants from hobby lobby, they cost about 20 bucks. Fake aquarium plants can be expensive. Make sure your fake plants do not have metal in them. Metal leaches into the water as it rusts and will be fatal to your fish.

Step 5: Finishing Up

I replaced my top brace, painted the stand black, and shined it up a bit. I also made a hood out of some scrap plywood.

Step 6: Add Water

After the silicone has sealed completely fill the tank with water. If you are new to fish keeping read up on the nitrogen cycle, it's imperative to keep your fish alive.