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I made this background for my 120 gallon aquarium. It was actually pretty easy to make but it was very time consuming and messy. I'd definitely do it again though! It was a lot cheaper than buying a custom made background and you can make it exactly how you want it. This instructable will walk you through how to make one of these backgrounds. You can adapt this to any size tank. The materials I used are fish safe.

***Note: This is only for FRESHWATER aquarium use!!!

Step 1: INGREDIENTS

*Note* This is an installed, permanent background. Be sure you are willing to dedicate a tank to this background and plan on it not being ready for fish for several weeks. Don't buy your materials and your fish at the same time unless you have a spare tank. The key is to not rush this!

You will need......

  • An empty fishtank
  • Owens Corning pink insulating foam sheets
  • 100% silicone (it is very important to use 100% silicone with NO additives. Avoid any silicone with mildew inhibitors etc as these types of silicones are not fish safe. Oh, and get more silicone than you think you need, you will use a lot!)
  • Serrated foam cutting knife (I used my kitchen bread knife ha ha)
  • Bamboo skewers
  • Toothpicks
  • Drylok hydraulic cement (my 120 gal background took 3 buckets)
  • Liquid cement color pigments
  • Cheap, disposable paintbrushes
  • Drop cloth (or two or three or four!)
  • Tape measure and ruler
  • Marking pen
  • Wire cutters

Step 2: Plan Ahead

Consider the type of fish you plan on keeping and plan your background accordingly. You want to create a happy environment that your fish will enjoy.

  • Measure the back of the tank and cut a piece of thin foam that size. For larger tanks like my 120 gallon, I had to build the background in two pieces. For smaller tanks, this is not an issue.
  • Dry fit the foam sheet(s) into the tank and trim where necessary.
  • Plan where your filter intake and heaters will go! Build the background according to where these areas will be. I planned a little alcove around the filter intake.
  • Plan on continually dry fitting your background into your tank as you build it. The last thing you want is to get it all ready and it not fit! I'm not saying this happened to me or anything...

Step 3: Start Carving!

I carved my rocks as I went and kind of let the background form itself. Keep a general plan in mind but don't try to control it too much. Have fun with it!

  • Using the serrated knife, start carving pieces of pink foam into shelves and rocks as desired.
  • Keep in mind that the cement will fill in a lot of crevices in the foam so don't worry about perfection. The foam is merely the base.
  • To create larger protrusions, stick blocks of foam together with silicone and bamboo skewers. Let the silicone cure and then carve away what you don't want.
  • I looked up images of rocks (especially slate) to inspire me.
  • If you like the slate look, see the above pictures to see how I carved it. You can do more rounded or blocky shapes too. Whatever you do, it'll look great once the cement is on it.
  • This should go without saying, but it's really easy for the knife to slip and cut your hand so be careful! You will need all your fingers later to install the background.
  • Carve your textures deeper and more drastic than you would think. The cement will fill them in a lot!

Step 4: Start Putting It Together

I used bamboo skewers and toothpicks to support and strengthen as I built. This will hold the pieces in place as the silicone cures. Use wire cutters to trim the skewers when necessary. I wanted to graduate my background as it went so it came out farther at the bottom and tapered at the top.

Important: I have had a few tragedies with fish getting caught in store bought aquarium decorations. I kept this in mind as I carved. You don't want to create any death traps for your fish. Avoid small sharp holes. The fish need lots of room to swim. Remember you can use the concrete to fill in any gaps you don't want.

Step 5: Build, Build, Build!

Keep building and fitting. I built my "rocks" onto square sheets and then trimmed a rough organic edge at the top.

Step 6: Cement Time!

This part is very messy. I used an old sheet as a drop cloth. This step is very time consuming. Allow several hours between coats and you will need to do at least four.

  • Mix the cement in small batches. It sets up very quickly. I used extra water (2 parts water to 3 parts cement) so it washed over the foam nicely. Mix the cement with cold water to increase the working time. You only have a few minutes to spread it before it starts setting up.
  • Coat the background two to three times with cement.
  • Let the cement cure. If it feels cold to the touch, it has not cured yet.
  • We will add pigments in the next step.

Step 7: Add a Color Layer

  • For your fourth coat, use cement pigment to stain it a nice dark color.
  • Paint the entire background with the dark colored cement. Mix it watery so it gets in all the crevices!
  • We went back in and touched up some areas with brown colored cement to add interest.
  • Let this dry.

Step 8: Drybrush

  • Your last coat will be using the original uncolored cement.
  • Lightly dry brush the cement over the background to bring out the texture. Only use a little bit, this is just for looks.
  • Leave the recessed areas dark. This creates dimension.

Step 9: Rinse, Repeat....

  • After the background is completely dry, give it a good rinse to get rid of any residue and dust. Let it dry again.

Step 10: Install It

It is important to create a good seal to the glass. You don't want any fish getting back there. Kuhli loaches, I'm talking to you! Use lots of silicone to adhere the background to the wall of the tank. Brace the background with foam while it cures. Let it cure overnight.

Step 11: Fill the Seam

If you had to build your background in more than one piece, you will have to seal the gap while it is installed in the tank. If not, you can skip to the next step!

  • Mix up some more cement and fill in the gap. It took two thick coats to fill.
  • Clean up any excess cement off of the glass.
  • Don't worry if it looks bad, it'll look good once you fill the tank.

Step 12: Seal and Paint

  • Run a bead of silicone along the edge of the background. If you get it on the glass where you don't want it, a little rubbing alcohol will soften the silicone enough so you can scrape it off.
  • If your background has an open area like mine, paint the back of the tank with a desired color. I just used acrylic craft paint because it's easy to scrape off if I get bored with the color.

Step 13: Setting Up

  • Fill the tank and get the filtration running.
  • Test the pH of your water before adding it to the tank. I have a high pH of 8.2 here, but it is consistently 8.2.
  • It is important to monitor your pH levels to make sure the background doesn't cause a spike.
  • I did not personally experience a spike or drop in pH, even after careful monitoring for three weeks.
  • DO NOT ADD FISH UNTIL YOU ARE CONFIDENT YOUR PH IS STABLE AND YOUR TANK IS CYCLED.

Any experienced aquarist will tell you the importance of cycling your tank before adding fish. If you are not familiar with the nitrogen cycle and fishless cycling please read how to do so before you add fish.

Step 14: ENJOY

Here is a picture of my tank after it has been set up for several weeks. It has developed a nice patina of algae. The fish love the little hiding places!

Thanks for your I nstructable. I think mine turned out ok. Didn't want anything big, just a nice rock wall. Made a spot on the bottom for my bubble wall to go, as well as a spot to hide my eheim filter.
<p>THATS AMAZING!</p>
<p>Very nice! I love how it turned out!</p>
<p>Props! Just inserted mine today, and waiting for silicone to cure before I begin the PH rinse. Started it back in June of '12!!! Got a little side tracked =)</p><p>Spring break found me with a little motivation. Hope mine looks as good as yours!</p>
<p>That is flipping awesome!!!!! I love the look!</p>
<p>My partner and I did this same thing and it turned out awesome! Will have to take more pics of it! This is definitely for only freshwater as the cement 'paint' you use would react with salt / calcium. Did research of that! Also just a tip we used acrylic paint to 'dye' the dry-lock and it worked perfectly and doesn't do anything to the water! For the Silicone I believe its Silicone I that is the standard and works perfectly! </p><p>Awesome job and we are definetly looking to do this again for other tanks makes it so much more interesting! </p>
<p>Awesome! Thanks for the input! Yes I agree, freshwater only! I should edit this DIY and specify that now that you mention it. Acrylic paint worked? That's awesome! I'll have to try that on my fourth background. That cement dye was so expensive, I'd love a cheaper alternative. Post a pic of your tank if you can! I'd love to see how it turned out :)</p>
<p>Here are the pics! We wanted it to sorta look like ruins so we hid the canister filter intake outake in a tower in the back corner with holes, and the a dragon mouth to spit out the water. We added an airline into the pvc to add air since the water wasn't churning enough originally to give them air. :) </p>
<p>Ohmygosh how awesome!!!!!!</p>
Water has been in for 3 days its still cloudy i also had a few fish die. Can you help me figure out whats wrong?
<p>For anyone else reading this, it was probably due to Ph. Concrete is acidic. </p><p>You have to soak/rinse the backdrop several times (rinse it thoroughly before putting it in the tank and even then I'd completely fill and empty the tank 10+ times to be sure), then cycle the tank as normal (look up fishless cycling online). </p>
<p>You have to chaco ammonia nitrates and nitrites</p>
<p>Check*</p>
<p>Check Ph and you have to cycle your tank. If you do not know what that is look it up. More so go to your local fish store and ask. NOT PETCO OR PETSMART!</p>
<p>I have been waiting a long time to start this project. Heres what i have so far. I grabbed some inspiration from you. </p>
Could you tell me exactly what kind of glue you used. Our foam floated to the top.
<p>Wouldnt be glue, 100% silicone cured for at least 24hrs</p>
<p>Your backdrop looks ace. I'm going to have a crack at it too and just wondered if I have to use the pink insulation foam or can I use some lumps of polystyrene I have at home instead? Thanks SS</p>
I was inspired by your 3D tank that I took it upon myself to try and do the same. I love it! I went with a slate look and even took it one step further by adding an under water sand waterfall! Thanks for the inspiration!!
<p>getting ready to upgrade to a larger tank and thinking about doing something like this. I'm curious though, has anyone ever done an underwater cave filled with air? I've already figured out the plumbing to cycle the air and keep the water level constant. The cave would be sealed to the back corner so you could see into the cave from the side of the tank. I can't think of a reason this can't or shouldn't work but I haven't found anything like this on the web so maybe I'm missing something?</p>
<p>I am going to make this for my stand to cover up the ugly metal. Hope it works good.</p>
<p>The best diy aquarium back drop I've seen. </p><p>Great job. </p>
<p>my b4 and after </p>
Can i use normal cement for coating and foam from my electronic packing device..thanks???
<p>hey guys love the idea, one problem im from south africa and we dont keep the same cement and im lost with what to use as im worried i poisin my fish. can you suggest a product in south africa that would work.</p>
I'm currently working on one! small run for a 30 gallon. 55 gallon is next! I'll post some more photos when its finished! I just started the cement coating!
<p>How did yours turn out? I have a empty 30 gallon tank now. And I just started thinking maybe this is the way to go. But I have 0 artistic ability. I'm just worried that mine will end up looking like a huge blob... Do you have more pictures? From what I see there, it already looks awesome!!!</p>
Thanks! I definitely would recommend this. Don't sweat artistic ability, just make it yours! I haven't finished mine yet, I'm self employed and got a little busy, but here's a photo of where it is now with 1 concrete coating.
That looks fantastic! Please keep us all updated as you go. Both this instructable and yours are amazing... I'm just not too sure I can do this. I can put a transmission in a car or rebuild a computer. But when it comes to stuff like this I just don't seem to possess the abilities. I love to watch others though! So y'all keep up the good work! And if you are close to North East Texas, I'll pay you to build me one ;-)
I made an enclosure for my bearded dragons like this a few years ago. Same process except for one slight modification... I didn't like the smooth cut edges of the foam, so I sprayed the foam with hair spray and then quickly torched the whole wall. It went up quickly and we knew it would, so there was a fire extinguisher handy. Afterwards, I applied the concrete and made a desert scene much like your cave system here. Very nice work. :-)
Coulda just used a heat gun
<p>That's an interesting technique! You should post a pic if you can. I'm very interested in how that turned out!</p>
<p>i love this in the pet store they can cost 50-150 dollaers </p>
How many days should I let it sit in the tank with water, until emptying and refilling. So on and so forth. Or do a couple a day. Or does it benefit better by letting it sit w day or two then drain and refill?
How long and how many water changes does it take to get the ph back to normal? Mine is gone way up and so far have done 10 changes
<p>My pH did not change. What kind of cement did you use? I don't have any experience with a pH spike like that but I am sure it'll take some time before it leaches out and stabilizes. Maybe someone else here has better input? My pH didn't change but that may be because I normally have a higher pH of 8.2.</p>
I used the same drylok and dye as you. My tap water is soft and between 7.2-7.6
<p>Ok so the cement must have a slightly higher pH than your tap water. How long have you had water flowing in the tank? It may take a few weeks to bring the level down. It's worth the wait though.</p>
<p>I saw this concept once before. This is beautifully done! Love it!</p>
<p>Thanks! It was a lot of fun. I've actually done two more since this one. I'll have to post pics.</p>
Sigh..... See now I wish I lived near you, owned a house with a nice finished basement/rec room just to have you make one for me.... Or rather, guide me along.<br><br>I got this whacky notion of doing a techie one..... With el-wire, led lights and maybe even moving parts...... It's the geek in me.<br><br>Picture a sci-fi spaceport!<br><br>Great.... Now I have to make two of them...... One line your &quot;marianas trench&quot; design AND a sunken spaceport......<br><br>:)
That would be super fun!
<p>Very cool, nicely done! And you did a great job writing this instructable. I've been wanting to make one of these for a while but haven't gotten around to it; This is great motivation for me. </p>
<p>Thank you! Let me know if you have any questions!</p>
<p>Thanks, I will.</p>
<p>Very nice result and instructions.</p><p>To get rid of the mess when cutting the styrofoam, use a hot knife. </p><p>I quickly found this one on youtube, but lots of others exists. You can even make it yourself, just by using the right kind of wire, and a common power-supply.</p><p><iframe allowfullscreen="" frameborder="0" height="281" src="//www.youtube.com/embed/W9j2iSWn1dk" width="500"></iframe></p>
<p>I think I need to pick one of these up! I tried to modify a Radio Shack soldering iron by welding a copper loop to the end but it just simply didn't get hot enough.</p>
I was a bit lazy when I mentioned the wire solution, and unfortunately left out some critical facts. Sorry. A soldering iron simply won't transfer enough heat to the wire, so you need the wire to become hot by itself.<br>For this to happen, you should use a wire that have a build in resistivity thats higher than that of copper, constantan wire (I'm sure it's available at Radio Shack) is an often used choise. You then need a common power-supply where you can adjust both the voltage and current. A few volts should be enough, and then adjust the current until the wire is hot enough to cut the styrofoam in a nice flowing movement. (A friend of mine used an acoustic western guitar string, 0.10mm, in worked just fine). <br>Note, if you max out the ampere dial, try again with a slightly higher voltage, till it works. <br><br>To easily cut, get a U shaped string saw, and strap the wire in where the string saw should be. Be sure to insulate the ends, as that kind of saws are usually made of metal. Use normal copper wire from the constantan wire to the power-supply. For inspiration, I'm adding a picture of a more pro, but home-made, cutter.
<p>So creative... Hot wire foam cutter could more useful in your project. </p><p>An example: </p><p>https://www.instructables.com/id/Hot-Wire-Foam-Cutting-Nunchucks/</p>

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