In this instructable i will show you how to make a cheap, and easy aquarium background. I use it in my 10 gallon nano saltwater tank, but you could easily make it to fit even the largest aquariums. Infact, i might even make one to fit my 55 gallon.

Step 1: Tools & Materials

Tools
1 Utility knife
or 1 circular/jig saw with a carbonfiber blade, or copper cutting blade
1 tape mesure
1 5 gallon bucket
2 towels
1 flat working surface
1 sun
1 greese pencil
1 builders or drywall square
1 peice heavy (40-60 grit) sand paper

Materials
1 Sheet of plexiglass 1/8-1/4" thick
1 tube Aquarium Safe silicon. (very important)
1 bag of lava rock

I already had the plexiglass, but i estimate the cost of the materials at under 15$.

Step 2: Measure & Cutting

MEASURE TWICE CUT ONCE!

Use your tape measure to figure the Inside dimensions of your aquarium. You will want it to be long enough so it can anchor itself in your substrate. Using your grease pencil, mark out the dimensions on the plexiglass using the two existing sides, so you only have to cut two times.

Use your builders/drywall square to acheive a straight cut with the utility knife. Press relitivly hard to get a nice, deep score in the plexiglass.
Once it is scored, position the score line (the score line being up) on the end of a table, counter, or work mate. With one strong movement, press down on the end that is hanging off of the table to achieve a clean break. Repeat with other score and proceed

Step 3: Measuring Some More

As you can see, in my aquarium, i am using an Under Gravel Filtrations system. That leaves me with two uplift tubes, and two powerheads that are very close to the back of the tank. I will not be able to place lavarock in these spaces. Other obstructions can be the uplift tubes of a hang on back filter, or heaters. (if you have no obstructions skip this step.)

Using your grease pencil, mark out where these obstructions will be, and remember not to silicon rocks in these spaces.

Step 4: Cleaning Your Rock & Plexiglass Prepping

Chances are there will be dirt, and quite possibly even mud, so you will need to wash your rocks extremely well.

Take your five gallon bucket, and fill it halfway with lavarock. Take your hose out to the street and fill your bucket. Churn the contents with your hand while still filling the bucket, letting all of the muck overflow. Empty the water and repeat. You will have to do it many times. Rinse untill you think the water is clear, and then rinse it three more times. You cant be too careful.

Spread your towels out in the sun on a flat surface in the sun, and spread your rock out amoung the towels. Every 15 minutes, turn the rocks around so they dry. (the silicone will not adhere to wet surfaces)

Now to prep your plexiglass. You want to rough up the side with your heavy grit sand paper to make the silicon adhere better. The sanding creates more surface area to bind to, and gives the silicon a place to anchor itself for a better hold. Dont overdo it, you want to sand in all directions, and have many distinguishable "channels" for the silicon to grab on to.

Step 5: The Boring Part

Now sit down at a table, grab a coke/water/beer/bag of human blood, and get comfortable, cause your in it for the long haul. You want to apply the rocks on at a time with a dime-nickel sized dab of silcon and apply it to the sanded side. Get creative, use the wierd looking rocks, use different colors sizes and depths to make it your own.

Step 6: Fin!

Insert it into your tank, and anchor it in the back into the substrate, step back, and admire your work. Heres a picture of the finsihed tank, and Shamus, the yellow tailed damsel (i didnt name him, my girlfriend did...her word is law *cracks whip*)