Here's a little Do-it-Yourself project. It's basically a background with a network of caves built-in, and covered w/ lava rock, so it blends in w/ the tank.
The following materials are required: Brown Vinyl guttering, hopefully you can find some identical to the style used here. I found it at McCoys Building Center, Home Depot uses a different type and I'm not sure it would work, but check around, you should be able to find something similar. Tubes of Black, Aquarium-safe silicone in Caulking tubes, these are available for less than $4/tube at www.bigalsonline.com and the manufacturer is All-Glass Aquarium. Caulking gun, small roll of Black Vinyl lawn edging, Drill,a 2 ½" holesaw, rubber dishwashing gloves, bag of black landscaping rock (golfball size is best), hacksaw or jigsaw. The total $$$ for this project, background is 6 feet longx15"high, was $7 for 2-10ft gutter sections, $5 for the lava,$5 for the edging, and $16 for 4 Silicone tubes. Under $35 for the completed project, although it will take quite a few evenings of cutting, gluing, and swearing. But looking back, I'm VERY glad I did the project, the fish love exploring the caves, and the tank looks much more authentic, especially after algae growth is prevelant. This pic is one of the pieces I use, it's brown vinyl guttering material, and the underside is white.
I drill small 1/32" holes and move the bit side-to-side to make small slots to run zip-ties thru, I use the zip-ties to connect the gutters together, and use a 2&1/2" holesaw to cut the cave openings. It's entirely up to you how many holes to drill, I spaced them close together (to be divided later) and then put another pair of holes further down the gutter, this provides alot of seperate caves that will create alot of different hiding places in your tank.
I then cut out pieces from a roll of black vinyl lawn edging (the stuff used to surround flower beds, etc.) and use them to divide off the caves. Using aquarium-safe black silicone I glue the dividers in place.
Here's the completed view of the dividers.
Here's the outside face of the gutters, prior to gluing the lava rock.
Now the time consuming part begins, I but a thin surface layer of black silicone on the entire brown surface, to give it an overall black coloration to match the black lava rock. You could use the maroon lava as well, but the color wouldn't blend as easy. Just put a small amount of silicone on the flatest side of each lava piece, and put together your rock puzzle. This is one of those projects you can do while watching TV, at least it won't seem so time consuming then! Be sure to wear dishwashing gloves, this stuff gets pretty messy!
Project's almost finished, I still have a few little gaps to fill in. I have an overflow that goes between the two backgrounds. The cool part about this project is it gives the fish about 24 caves to cruise around in, it's lightweight (about 20lbs/side), and I connect it to the tank w/ zip ties hooked thru the holes drilled in the top of the ACRYLIC tank, if you put it in a glass tank you'll have to divise some hook arrangement, or possibly drill small slots in the plastic edge used to frame the tank. If it needs to be removed I just cut the 4 zip ties and presto, it's out!
Here's an installed picture.
Here's one with the fish.