Make a 3D Aquarium Background




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!!!


*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.

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!

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99 Discussions


4 years ago

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.

2 replies

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!

Awesome job and we are definetly looking to do this again for other tanks makes it so much more interesting!

3 replies

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 :)

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. :)


17 days ago

I absolutely love this! However I am curious if its poss. To make it non-permanent?? How could I weight this down.


Question 10 months ago on Introduction

What is the thickness of the Owens Corning pink insulating foam sheets? should it be a certain thickness to support the cement & any substrate pockets for plants Etc? Thank You for Your advice & instruction.


Question 12 months ago on Step 6

I let the quick Crete cure between layers and when I went to rinse it the quick Crete just came off. What am I doing wrong?


12 months ago on Introduction

Be careful, everyone! I went to the ER last night to get stitches in my thumb using this guide. If your using a big knife like I was, just go slow and make sure you know that if your hand slips, it won't cut you


Question 1 year ago on Introduction

Hi there u are awesome. I had a question: what if i want my rocks to look in red colours,what should i use?
2nd,can i use a normal cement powder with water? Please hslp appreciated. Thank u so much. Singapore

1 answer

Answer 1 year ago

Acrylic paint is safe to use in a fish tank. I have used it on several tanks over the years mo problems.


3 years ago

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!

4 replies

Reply 1 year ago

Hi there..what if i want my rocks in red colours what should i use. Thank u

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!!!


Reply 3 years ago

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.


Reply 3 years ago

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 ;-)


Question 1 year ago on Introduction

Hi there I have searching for a good way do make my own I love this. Now what do you think is better Styrofoam or the pink?

1 answer

Answer 1 year ago

I intend to build my background too n i will go fir the pink