I have been trying to figure out how to make a background that I would like for quite some time. I finally came up with this. I used fluorescent light egg crating, aquarium safe silicone, and a bag of garden river rock from Home Depot. I had to cut the egg crate into 3 pieces to make it fit into the tank because of the glass support across the top middle of the tank. Probably just as well because the egg crating breaks fairly easily and with the weight of one solid piece I am sure I would have broken it before I got it in the tank.

Looking back I would probably use a sheet of 3/8" plexiglass cut into 3 pieces instead. (though it would cost significantly more). I have been really pleased with the way that it looks and my fish seem to really like it. They swim a lot higher in the tank now. It also seems to give the tank a lot more depth than not having a back. If you look in the bottom right of the picture, you will see a network of caves I made using the instructions by James Kuhn. Anyway here's the steps I used.

1. Wash the river rock thoroughly and let it dry. You want to be sure that you get off as much of the dirt and dust as possible, or it won't glue well.



2. Measure the inside dimensions of the back of the tank and cut the egg crate/plexiglass into manageable sizes that will fit into the back of your tank.



3. Scratch up one side of the egg crate/Plexiglas pretty good. You can use sandpaper, I used a little handheld sander. This gives the silicon much better hold on the backing.



4. Be sure to wear gloves while working with the silicon. I bought a pack of 20 pair for a couple bucks at Sally's hair suppply. Apply a liberal amount of silicon to a section of the Plexiglas/egg crate (make sure to use the side you scuffed up). Be sure not to do more that you can place rocks on in about 5 -10 minutes. You don't want the glue to start drying before you have your rocks in place.



5. After you have applied glue to a section, grab a hand full of rock and drop it in the silicon. This will leave little gaps all over the place, but I preferred it to exactly placing the rocks because it looked more natural.



6. Repeat 3 and 4 till you finish a whole sheet. Be careful when working on the side that will be agains the tank. You don't want the rocks positioned so that they scratch the glass.



7. Carefully turn the sheet over. All the rocks with a poor grip should fall off the sheet now. Turn the sheet back over and run your hand over it so that you knock off the last of the loose rocks.



8. Now you can go back and fill in the gaps with the leftover rock that you have.



9. I let the pieces stand for 48 hours, then I placed them in a big ice chest to let them soak for another 48.



10. That's it. Put them in the back. I am sure there are a ton of different ways to use this. You could use different rocks. One idea I have had is to get some lengths of PVC, cut them in half lengthwise, Cut some varied holes in them, glue them to the plexiglass, then cover it with rock. Instant caves.