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Faux Stone Aquarium Background

By keno | February 3, 2010

Faux Stone Aquarium Background

I’ve had a number of folks asking about my faux stone aquarium backgrounds. I was planning on posting this article once I had a full set of images to go with it, but since there is a lot of interest, I am going to post this now. Please note that most of the steps do not have images, due to a mishap with my digital camera.

Disclaimer: Exercising reasonable safety precautions when performing the steps described in this article. This project involves the use of electricity, heating tools, cutting tools and chemicals. This article was created simply to share what I’ve done. These are not verified instructions. I take no responsibility for any harm caused to you, others, or to your property as a result of the info contained in these articles. I have not tested any of these supplies with saltwater aquariums.

Materials Needed:

Tools Needed:

I am going to describe how to make a faux stone background, but remember this technique can be used to make standalone faux stones or groupings of stones, etc. It’s up to your imagination on what you can do. For my 200 gallon aquarium I am planning on making not only a faux stone background, but also standalone faux stone features. That’s for a future article.

Please Note: (Some styrofoam boards come with a plastic sheet that covers them. REMOVE the plastic sheet before starting)

The 1/2″ thick styrofoam board is used to cover the glass or acrylic back of your aquarium. You would measure and cut this to size. I like to leave a 1/4″ gap on the left and right sides. You need to make sure that you can fit the styrofoam board into the aquarium now. If it is tight now, trim it smaller. It will only get wider and taller once you add the concrete mix to the board. For example on my 55 gallon aquarium, I have a bar in the middle of the top opening, so I needed to cut my 1/2″ styrofoam board into two pieces to fit into the aquarium.

Creating the Faux Stones

Now comes the fun part and creative part of this project. You need to know what you want the finished background to look like. Do you want it to have flat stones like you would see if you had flagstones laid flat on a wall. Do you want to have rounded river stones? Me I like the look of stones laid on edge with some gaps between them. I can then use those gaps to place plants like mosses or Anubias. The roots from the mosses and Anubias will then grow and attach themselves to the background and creep down the surface.

Now take your 1/2″ styrofoam board and mark top and bottom on it with your marker. You will also need to know the depth your substrate is going to be. Once you know the depth then measure from the bottom of your styrofoam board up that distance, say 3 inches and make a mark on the styrofoam. Using your straight edge draw a line across the bottom. The reason I do this is to indicate where the bottom row of faux stones are going to be placed. It’s not a hard rule that all of your stones must be on this line. Some can be a little above some can be below. I like to have the space under the faux stones for the substrate materials.

To cut the faux stones I use the 2″ styrofoam. I will take my straight edge and make a straight line on the foam. Usually I measure about 3 inches from the edge of the foam to where the straight line is drawn. If you want less stone face use a 2″ spacing. Deeper stones, a 4″ spacing. The purpose of the straight line is to give you a flat surface when you silicone the styrofoam stones to the styrofoam background. To make your stone shapes I just freehand the stone shape. I will use “real” stones to act as an inspiration. I will then just start drawing curved lines. Once you have your lines drawn, I will then either using a sharp large knife or a heated cutting tool, cut at the straight line and separate this piece of foam from the larger board. I will then cut out the individual stones. Having a heated cutting tool helps at this point. I picked up one at a local craft store ACMoore for around $20.00. It’s not super fast to cut with, but you don’t end up with all those little foam pieces that get everywhere. Make sure you are cutting (melting) the styrofoam outside, you don’t want to be breathing the fumes. There are larger heated cutting tools that look like knifes, but they are also more expensive.

Ok, so now you have these faux stone cutouts and they are flat on top, flat on bottom and flat on the back with some type of curved front. Doesn’t look like a real stone. This is where my wife (who has her own stamping and scrapbooking business) stepped in and said, “you aren’t going to leave them like that are you”? So she took the knife and began hacking away at the foam removing the sharp edges and put in gouges, etc. So now I had some hacked up faux stone cutouts. Just make sure you leave the backside of the stone (the part that was made with the straight edge) flat. This will make it easier to silicone to the 1/2″ styrofoam board. I could kinda see a real stone hiding in the hacked up styrofoam. Doing this will also give you stones that are all not the same width, etc. So now you can proceed and make a bunch of these hacked up styrofoam stones.

Laying Out the Design

Once you have a bunch of stones cutout and hacked, start laying them on the 1/2″ rigid foam board in different patterns to find one you like. Remember to account for items that will be in the aquarium later. For example, I have a wet/dry sump on my 55 gallon aquarium which requires an overflow box that sits in the aquarium so I needed to make sure the stones would not interfere with the placement of the box. Also on my 20 gallon aquarium, I needed to make sure that that I drilled a hole through the styrofoam for the intake pipe for the filter. Once you find a design you like then it’s time to start siliconing the individual faux stones to your 1/2″ styrofoam board. This is why I purchase the large tubes of silicone and used my caulking gun. Just remember we want just pure silicone with no additives. Granted the silicone will be sealed later by the epoxy, but why take chances. When you start siliconing the faux stones to the 1/2″ board keep the silicone between the flat back of the faux stone and the 1/2″ rigid foam board. You don’t want silicone getting on the face of the 1/2″ foam board. Once you have all the faux stones in place let it dry for 24 hours.

First Coat of Concrete

In my materials list, I listed Quickcrete Blended Mason Mix. On my first DIY Faux Stone Background I used regular concrete mix. The problem I had with it was the small stones. It was hard to work with the stones and I ended up sifting them out of the dry mix. The Quickcrete Blended Mason Mix has no small stones. I ended up purchasing an 80lb bag at my local hardware store for about $5.00. This is where I also picked up the concrete colorants. The ones I purchased were liquid colorants. For the first coat of concrete you want to make a mix that is thin. The purpose of this coat is to give the future coats something to stick to. You want to make sure you wear some type of disposable or rubber gloves when you apply the concrete mix. Just brush it on. It is not going to look like much at this point and you may still see the foam through the concrete coating. I don’t add any colorant on the first coat, since you will not see it later. Make sure you coat everything front, back, sides, top, bottom and between all the stones. Just keep working it in. Once you have it covered. Let dry for 24 hours.

Second Coat of Concrete

Here is where you would add the colorant to the concrete. You would mix the concrete to get a thin paste and add your colorant. I like to add the red color on the first coat. With this coat you are going to completely cover the entire face side of the foam project. Don’t worry about the backside of the 1/2″ styrofoam you will coat that in the last concrete layer. Make sure you use your gloves and a disposable brush. You want to make sure that you get the concrete between the stones and cover all exposed surfaces. You want to keep the concrete coat thin. If you put it on too thick you may get cracking. Once covered let dry for 24 hours.

Next Coat of Concrete

I now mix another batch of concrete and colorant typically the black. Once again using your gloves and a disposable brush I apply the concrete to the surface. This time I try to not completely cover the red coat. I will apply the black color in a stippling fashion, which is more of a quick tapping of the brush to the surfaces. When done once again allow the concrete to dry for 24 hours.

Additional Coats of Concrete

I will typically add one more coats of the buff color in the same fashion as the black color. With this coat I will also go and coat the sides and flat back of the 1/2″ styrofoam board. When done I let this dry for at least 48-72 hours. If you find you would like more black or red showing here is where you can make small batches of concrete in the color you want and stipple the surface. Once again after any touchups let dry for 48-72 hours. Once it has dried I will dry fit the background into the aquarium. If you need to trim or make changes now is the time before you apply the epoxy. Be careful when doing this as to not scratch your glass or acrylic.

Applying Epoxy

In this step you will seal the concrete of your faux stone background. I found reading on the internet that most concrete backgrounds will affect your water’s pH levels and you need to go through multiple aquarium fillings and drainings to stabilize the pH levels. I did not like the prospect of having to do this especially on my 200 gallon aquarium. I wanted something that was neutral and would not affect the water chemistry. In doing some searches on the internet I found a person who makes wooden aquariums. He was using the West Marine epoxy to seal the plywood. I figured if it worked for that it should work for the faux stone background. For the epoxy you need to purchase the resin and the hardener. I also purchased the pump kits. The pump kits pre-measure the amount of resin and hardener, so it just a matter of putting in the correct number of pumps for the resin and hardener as per the labels. For my first DIY faux stone background I purchased the 105 resin and the 206 slow hardener, based on the advice of the guy at the store. The 206 hardener was not the one I wanted. It worked, but it had some properties to it that I did not like. One was that it took way too long to harden and second it would blush, especially if you put on a second coat. The 207 hardener will dry clear, and doesn’t have the tendency to blush. What I have also done is I only apply one coat to avoid any blushing.

So once you have the resin and epoxy mixed, I put on my gloves and get one of my disposable brushes. I will then work the epoxy all over the front side of the background with the background laying flat on the ground. At first it will seem as if you are not getting much coverage. This will change as you get more of the background covered in epoxy. You want to make sure you get into all the nooks and crannies so that all of the concrete is covered. I will mix small batches of the epoxy and typically have to make several batches to cover the background. You may find that you will get areas that will seem to pool the epoxy and the epoxy will get milky in color. I will use a second and sometimes a third dry brush to work those areas and remove as much of the excess epoxy. The other thing you will notice is that the background will turn darker in color, this is normal. Also when the background dries it will look shinny and darker in color, this is also normal. Once you put the background in your aquarium and fill it with water, the background will lighten back up. Why this happens, I don’t know but it does. If you are not sure about the colors, you could make a couple of test pieces and coat them with the epoxy and then put them underwater to see how they look. Once I completely cover the front of the background, I will let it dry for 24-48 hours. I will then turn it over and coat the back and the sides of the 1/2″ styrofoam board. Watch for drips. I find if you have the background at an angle when you coat the back and sides most drips will run to the bottom which will be buried in your substrate anyway. Once you have coated the back of the background I will wait 48-72 hours to allow the epoxy to fully cure. As I stated earlier, I only put one coat on the front and back of the background.

Mounting in Your Aquarium

To mount the faux stone background in your aquarium I found it was easier to lay the aquarium on it’s back. I only clean the glass or acrylic with clean water and buff the surface dry. It needs to be completely dry or the silicone will not stick. I will dry fit the background one more time. I will then lift the background and apply a generous amount of silicone to the background and the aquarium surface. I will then press the background to the aquarium surface making sure to push down and squeeze the background to the aquarium surface to get as close as you can. You don’t want to have any gaps that fish or other inhabitants can get into. I will now leave this for another 24 hours.

Adding water

Once the silicone is dry, I will add water to the aquarium and see what the background looks like. I also will run the aquarium for 24 hours with just water and the background. After 24 hours I will drain and then add my substrate and plants and refill. I have done this on 2 aquariums at this point and the water parameters have not been affected by the epoxy coating.


You will see in these images just how light the concrete looks before and after coating with epoxy and what it looks like once it is underwater. Also note that there are two backgrounds a left and a right for my 55 gallon aquarium. I also included a couple of images from my 20 gallon aquarium which also has a faux stone background.

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Topics: DIY Projects | 3 Comments »

3 Responses to “Faux Stone Aquarium Background”

  1. gilbert britto Says:
    May 5th, 2011 at 2:04 pm

    beautiful work!! very informitive and answered my main question regarding treating raw concrete diy backgrounds, although you can do it withoout the epoxy resin, i like your method much better, i feel your diy bk’s last longer and could be the answer for those who have fish that tend to nibble the untreated concrete down to the foam!

  2. Serghei Says:
    April 4th, 2012 at 6:03 am


    Thanks for your detailed description.
    I would like to clear some things.

    Is it suitable to make the Faux Stone Background significantly narrowwer(smaller) then the aquarium itself. I’m just thinking to put some kind of “rock” in the corner of my 200 liter aquarium. However I’m not sure if it is fine to make the size of the backgound the half of the height and the 1/3 of it’s length. I want it to be just in the corner.

    And if I’ll put it on silicone will it remain there on the bottom? Because the styrofoam itself is lighter than water so it tries to float.

  3. Chris Lehenbauer Says:
    September 5th, 2012 at 6:30 pm

    Beautiful backgrounds!! I’m in the process of building my own background now, similar to the way you’ve done yours. My question is how did your set ups in the acrylic tanks make out sealing it with silicone? I have read that the silicone won’t adhere to the acrylic long term. Did you have any issues? I’ve bought some Gold label pond and Aquarium sealer which is supposed to to the job. But at $30 per tube, a $5 silicone tube is better for the future!