3D Aquarium Background




This instructable will show you how to make a cool 3D background that goes inside your aquarium and looks like real rock but only weighs less then a couple of pounds.

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Step 1: What You Need

You will need the following

Razor or Box knife
Foam cutter or maybe electric knife (both are optional)
Ruler or other sutible straight edge
Marker (sharpie)
Tape measure
Caulking gun
Hot glue gun (optional)
Cheap paint brushes, all sizes
Wire brushes
small buckets
Tarp (optional, to keep mess at a min)

1 inch pink foam board (the kind you buy at Home Depot or Lowes)
100% Silicone for caulking gun (GE silicone 1 is what some recommend) I used the Red Devil Architectural Grade.  As long as it does not contain any mold or mildew inhibitors and is 100% silicone it should be fine.  The less ingrediants the better IMHO. 
hot glue (optional)
Cement or cement overlay
Cement coloring (optional)
Small finishing nails

Note on pink foam: There are two types, one has a plastic cover on it (vapor barrier I think?) and the other does not.  I bought the one that did not have the plastic on it.  If you buy the one with the plastic on it, you will have to peel the plastic off.   Also if you have access or want to buy more foam, you can buy several thickness of the stuff to help create different thick and thin layers.  I just used one sheet of 1 inch stuff and glued it together to get the thickness I wanted.

Note on hot glue:  I have read where some people used hot glue to glue everything together instead of silicone.  It is faster and it shouldn't pose a threat to your fish.  I used a combination of both, hot glue on the smaller pieces and silicone on the larger pieces.  Its up to you.  Trying to get hot glue to come out in the quantity I needed on some of the large pieced didnt work.

Step 2: Lets Get Started

First figure out if you want to cover just the back wall of your aquarium or more.  Also decide what you want your background to look like.  You tube has some great video of some that others have built and a google search also comes up with some great ideas.  I decided to cover 3/4 of the left side and the entire back wall of mine in a flat stacked stone style.  Some people do 3 sides as well. Use your imagination if your talented enough!

Step 3: Lay the First Layer and Pieces

Since I decided to do just one side and the back I measured the back first. I wanted the background to come to within 1 inch of the side I was not going to cover, this was because I wanted to taper the edge off and not have a sharp square end showing. I then measure the left side of the aquarium, I decided on this side I was only going to cover about 3/4 of it. Then cut your foam to fit. My 75 gal aquarium was 4 foot long on the back and I needed about 10 inches on the side. I measured 1 inch shorter because of the taper effect or the back piece. I also had to make my background in 2 pieces to get it inside and past the center support when I am finished.  I also cut the pieces 2 inches shorter so that there will be sand under the edge when I  am done. I started with the back corner. I first used the silicone and glued the two corner pieces together along with the first bottom "rock" cut out of the foam.  I used the finishing nails to hold it together until it dried.  Some people cut the rocks first and test stack them.  I just cut them out of the foam as I needed them.  I just used my imagination on the shape of the rocks.  This is where a foam cutter comes in handy.  If you dont want to make one or have one, just use an electric knife or even just a normal knife and cut and carve.  Remember rocks are not perfect but also remember small details will not show up after you coat the thing in cement.

Step 4: Continue to Lay Pieces...

Continue to cut and lay pieces in the desired pattern you want.  I cut the pieces and test fitted them before gluing them to the back piece.  I glued two pieces of foam together using silicone and the finishing nails for my bottom row and allowed them to dry.  Be sure to remove the nails after they dry.  After they where dry, I carved them to the desired shape.  Use the wire brushed or the razor knife to give them a rough look.  Use your imagination!

Step 5: Building Other Side

I had to cut mine in half to get it into the aquarium.  I tried to overlap pieces so that the seam would not show but I found this to be too restrictive when I went to test fit it and it would not go together.  I will have to fill in the crack later after its in the aquarium.  I tried to keep all the "rocks" lined up by laying it out on the table while building the other side...

Step 6: Final Test Fit

After you get all the rocks on it that you want, be sure everything lines up and fits together inside the tank like you want before the next step.  Sorry I did not have a pict of this step but I had one right before I finished it.   Also shows where my other filter will be.

Step 7: 1st Coating

This step is the first in coating your background.  You want to mix the concrete ALMOST water thin.  Sort of like runny pancake batter.  I used a concrete overlay called Ultra Tex U-91-1 but others have used regular concrete and the quickcrete overlay stuff.  What ever you use make sure its doesn't have a lot of stones in it or you will be sorting through it to remove them.  You want a smooth mixture and real watery.  This first coat will only be the base and give the additional coats something to stick to.  You dont have to add color to this coat but I was playing with the color mix to figure out what I wanted for the rest of them.  Once you mix it up just use the cheap throw away brushes and brush it on like paint.  I poured some on then used the brushes to spread it around even.  Dont worry if it looks a little thin, the second and 3rd coats make it thicker and cover what this coat doesnt.  Let this coat dry for at least 24 hrs, more the better

Step 8: 2nd Coat

The next coat should be a bit thicker.  About the consistancy of thick pancake batter.  Again spread it on with he brushes.  I suggest adding some of the color you want to this coat. I tinted this coat a lot lighter then the last. Again let this coat dry for at least 24 hours or longer.  The longer you let it dry the stronger it will be.  This coat went on a bit smoother and started filling in the gaps and small details.

Step 9: 3rd Coat

I mixed this coat about the consistancy of the first.  Add the color as well.  I wanted to paint it on using the brush in a dabbing motion.  I didnt want a solid coat because I wanted the last coat with the lighter color to show through. If you want to add more color then mix another batch of cement up with the colors you want and add it to this coat and repeat until you have the color and look you want.

Step 10: Test Fit...

After everything is dry, perferably at least 24 hours, I let mine dry a week, test fit it into the tank and try to plan how you are going to get it into the tank with silicone on the back without getting it everywhere.  I cut my spacers and some braces before hand when I realized that the background tried to slightly lean out on the top when it was in place.

Step 11: Clean the Tank

I didn t think you needed a picture of how to clean the tank.  Remove the background and make sure the glass surface you are going to stick the background to is clean as you can get it.  I used a razor blade to scrape all scum and hard water stains off mine.  Cleaner the better.

Step 12: Apply the Silicone to the Back

Next apply the silicone to the back of the background.  Dont be shy about this step and add lots of silicone.  Even though it has cement on it, these things still float like life preservers so dont get skimpy here.  I ran a 1/4 inch bead about every 1/4 to 1/2 inch across.  I dont know if it makes a difference but I ran the silicon lengthwise along the pieces.

Step 13: Install in the Tank

Now remember practicing in step 10 ?  Here is where it will pay off.  Install the pieces in the tank and brace/block/tape or hold in place till it sticks.  be sure to press on it slightly to help spread the silicone against the glass.  Allow to dry for at least 48 hours before adding water.

Step 14: Fill, Drain, Fill, Drian, Fill...

This step may or may not be necessary but needs to be considered.  After filling with water, check the PH level.  Let it sit for a day or two and check the PH again.  You may discover that your PH has climbed pretty high.  If it does, drain your tank and refill.  Do this as many times as it take to leach out all the PH raising chemicals.  I did not have to do this step but I included it because other on the net report having to do this.  I dont know if it was just the stuff I used or our well water.  Our well water is high PH to begin with but I let mine sit a week and didnt see any change in PH levels.  I have heard of people having to drain and refill up to 15 times to get the PH to stabilize.  Just check it and decide for yourself, you dont want to kill your fishies...

Step 15: Final Fill and Add Your Stuff!

After letting mine sit for a couple of weeks I felt confident to start cycling the tank.  I added my sand, started my filters and started a fishless cycle.  Its way easier on the fish to do a fishless cycle.  I also will be building a canopy to hide the lights and filters, maybe another instructible...

Step 16: After Several Months of Operation

After some requests, I added some photos of the background after it has been up and running for awhile now.  Has some algae growth on it and some black build up, not sure what the black build up is, some sort of algae I suspect, it does come off if I scrub it but I like the way it looks and the fish and trumpet snails dont seem to mind it and it doesnt seem to effect the water quality.  If it ever gets too built up with it I will just scrub it and let it begin again.

5 People Made This Project!


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96 Discussions


9 months ago on Introduction

Hi!! What type of paint did you use for the color?? Or is it just regular paint “sealed” up by the thin cement layer??


Reply 9 years ago on Introduction

You can do this with any size aquarium, just scale it down. If you had a 1 gal you could use some VERY thin foam to do this with, like 1/4 inch packing foam to make it look right.

Charley 68Rainh2o

Reply 2 years ago

Scale down? ??? I'm planning on doing this for a 750 litre aquarium! Gotta scale it up ⬆⬆


Reply 9 years ago on Introduction

yeah... we, unfortunately, no longer have any fish. but this is still cool and i will definitely try it if i get the chance anytime in the near future


3 years ago

Is this just styrofoam and the concrete mixter?


3 years ago

Is this sturdy enough to allow turtles to climb on it and bask? Anyone tried this with reptiles at all?


3 years ago

This is fabulous!!


3 years ago

Hey there,

Did you mix your concrete with sand?


Reply 3 years ago

LaylaT1, If I were going to put this in a saltwater or reef tank I would soak it for nearly a year doing weekly water changes in some other type of dark container (so no algae). This is based on people making "aragonite rock" which is portland cement mixed with crushed aragonite. It's commonly referred to as "homemade reef rock" and lots of people make it to replace live rock in their reef tanks and saltwater fish only tanks. You can look it up, but most people recommend soaking it for a long time and I would too.


4 years ago

Hiii i want to ask that if we use cement for colour so it will harm my fishes or not ?? Did cement can give any harm to my fishes


4 years ago on Introduction

Nice work .. Working with foam by hot water foam cutter really Artistic work and addicted !??


4 years ago on Introduction

Pulling up an old instructable here. Love what you did. How long did you have it in your tank? Do you still have it? How did it hold up to algae? Did you use this on a planted tank? Fish?

My thought was to create something like this with a planted tank and plant some ferns or something right on to the polystyrene. Curious as to how it will hold up over time though as I don't want to have to pull it out, scrape everything down, and rescape in a year ;)

Would love to hear from you if you still monitor this. Thanks!

2 replies

Reply 4 years ago on Introduction

I no longer have the tanK, moved on to other hobbies. The wall held up for about a year, I then had an outbreak of black algea, due to some plants I got. The black algea was a nightmare to get rid of. The wall can be scrubbed and cleaned with no problem. It seemed to hold up well before and after the outbreak.


4 years ago on Introduction

Hey, thanks for this cool Instructable! It's now 4 years later, and I'm wondering, how did it work out long-term? In particular, I'm worried about algal growth, which looks like it would be really hard to clean off (especially if you've glued it to the tank). Did you find yourself with a ruined tank and deep regrets several years later? Or are you still happy with it?

1 reply

6 years ago on Step 7

Latex Drylock works much better than concrete. It is durable and it won't raise your PH like the concrete will. You can tint it with concrete tint powder. It is widely used in the hobby and completely safe.

1 reply