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A DIY background for the Small Tank
by John D. Drysdale (a.k.a. throw_this_away)

To date most of the DIY concrete backgrounds that I have seen have been for quite large tanks (generally 50+ gallons). Why was that I always wondered? The first thing that came to my mind was that maybe it was really hard to get good detail using Styrofoam/concrete so it kept people from making smaller backgrounds. I'm here to say that you should not be intimidated the technique works on small tanks, and looks pretty good (it just requires a little patience)!!!

Water Window Bedrock Plant Terrestrial plant

STEP 1: Get Your Materials

I will start off by saying that I got all of my methods from the three excellent articles published on this forum. Though I did not follow any one of the articles exactly, I did read all three to get the general procedure, and just sort of did my own thing (I suggest you do the same). Still, the following was my list of materials

  • Paint Brush (3 inch wide)
  • Paint Brush (1/2 inch wide)
  • Quikrete Hydraulic Water-Stop Instant Plug Cement (Note this contains no fiberglass fibers. I used about 3 kg for my 2 15 gallon tank backgrounds)
  • Empty Yogurt container (for concrete mixing)
  • Newspaper (for the floor)
  • Black marker
  • Large toothed knife
  • Fine Penknife/box cutter (for finer cutting)
  • Sandpaper
  • Styrofoam cooler box (2 1/2 inches thick, large enough for your tank dimensions)
  • Aquarium safe silicone
  • Water softening salt
  • Black paint

STEP 2: Designing Your Background

So what do you want your background to look like? Do you want round rocks or jagged ones? Big rocks or small ones? In designing my background I looked at pictures of rock cliffs in books, browsed the designs of commercial aquaria backgrounds, and looked at some of the other DIY backgrounds on this site. I basically got a rough Idea of how I wanted my rocks to look and then I just made it up. My key design elements were to have my background look very 3D, with large cracks between the rocks and rock overhangs that would create cool shadowing effects in the final product. I also decided that my background would extend along the left side of my tank, so it would not just be limited to the back of the tank.

With my general design vision in place in my head, I cut my Styrofoam box to match the back/side dimensions of the tank. I then used my black marker to draw out the general rock shapes/cracks on the surface of this Styrofoam "wall." You will have to use some imagination at this point, but it does help you to see what things will look like once carving begins. Remember that for small tanks, the smaller you design your rocks to be, the more the tank will have an illusion of being much larger than it really is.

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Just an old Styrofoam cooler box. Pretty humble beginnings.
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A rough design sketch for the carving to come. This piece will go in the far left corner of the tank.

Of note, now is the time to figure out where you will be putting your filters/heaters/power heads. I made a chamber large enough for both my heater and filter impeller with room to spare behind each background. I also made sure to incorporate chamber vents to my rock design such that some of the cracks that I would carve between the rocks would lead to the heater/filter intake chamber.

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Red arrows show crevices that lead to the filter intake chamber. Blue shows the location of filter output.

STEP 3: Rough Carving

So now you have a wall of Styrofoam that needs shaping (note it is not yet attached to the tank). This can be a pretty fun step, but it is very very messy. I suggest you have a shop vacuum on hand to suck up the smaller Styrofoam bits, otherwise you will find that they will stick to everything including you. The next thing you know your place looks like it just had a fresh snowfall. My floor was totally covered when I was done carving.

Follow your original lines when cutting but don't be too strict about it. The more you cut and carve the more you may want to alter and shape things differently. Go with your gut feeling but always keep your greater design in mind, be it in your head, or on a picture beside you. Also carve your rocks with lighting in mind. How will lights and shadows play off the structure? Try to create as much depth as possible in your carving. It will take patience, but will pay off in the end. I also added to the depth of my background by carving separate rocks from spare chunks of Styrofoam and then siliconing them onto the greater structure later on (this is how I made the rock overhangs on each of the backgrounds).

Making Styrofoam Rocks

I always felt that real rocks combined with a Styrofoam background made the background look wellconcrete (unless you can find rocks that match both the color and shape of your carved rocks). With that in mind I decided to make a few rocks out of Styrofoam instead of using real ones. I carved them and then just siliconed them to the bottom of the tank (after coating them with concrete of course).

Carving caves and crevices in your background

Don't be afraid to cut cracks between rocks that go all the way through in places. If you paint the back glass of your tank black this will give the illusion of a very deep cave or crevice. I took the idea one step further and actually had a number of larger cracks that let into hollowed out caverns behind the Styrofoam. These caverns can serve as homes for your fish, and they help to minimize the water displacement caused by the background. Of note: remember that if fish choose to hide in such caves you will never be able to catch them. I wasn't worried in my case as I only planned on having shell dwellers in my tanks, but the caves could be problems for cave dwelling species. Just in case, I have left the glass immediately behind the caves unpainted, but covered by black removable shutters, such that I can see any fish in hiding might be cool for seeing the eggs of cave spawning species!

When carving caves, be careful not to cut away too much Styrofoam such that you sacrifice the structural integrity of the foam. Also be sure to leave enough surfaces for the Styrofoam/silicon to properly adhere to the back glass.

Making your background in pieces

If your background is a good fit, you will probably have issues getting it into you tank in one big piece. I designed my backgrounds in three major pieces for each tank, with 3-5 smaller overhang rocks to be added later. When sectioning your background do not make obvious straight vertical cuts. Try to cut them such that each piece will fit like a puzzle you don't want people to be able to identify where you joined sections.

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A view of some of the side caves in the finished
product. Siliconed areas were covered with black paint on the outer glass to give a cleaner final look.
These caves can be covered with black card to provide the fish with privacy.
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The red lines depict where separate pieces of the background were joined.

STEP 4: Carving Touchups

So now you have your basic rock structure carved. Is the time to give it a last once over. Use your finer penknife to add small details to the Styrofoam like surface indentations and rock imperfections. The more the merrier, but remember that details that are carved too fine will be lost when concrete is applied. You may also want to lightly sand any excessively smooth surfaces with rough sandpaper to aide in concrete adhesion.

Finally, use your shop-vacuum to suck up any Styrofoam particles/bits from the background as they will only cause trouble when you begin to apply the concrete.

STEP 5: Applying Your Concrete

I won't get into the specifics of this as it is well outlined in the other articles on this site but I started with one thin primer coat, followed by 4 more thin coats allowing each coat to dry in turn. Using many thin coats is preferable to a couple of thick ones as it allows you to maximize the level of detail, important in a small tank.

You will have to play with the amount of water that you add to get the desired consistency. Too little water and it won't paint on well (papa bear), too much water and it will dribble all over the place like soup (mama bear). You want something in the middle, you are striving for baby bear. For every layer I used only a brush never my hands. It might work for much larger backgrounds, but I will repeat, with a small tank detail is key, and using your hands results in loss of control, especially around cracks. You don't want to completely fill in what you spent so much time carving. Dabbing with the tip of your brush also helps to create a nice rough texture for the concrete.

In some of the finer nooks and crannies I used the smaller brush to apply the concrete. Again, be careful to not fill in all your carved detail. In the end you want to maximize detail without sacrificing concrete sturdiness/thickness. Do not be skimpy on the concrete or you will regret it down the road if it cracks and starts showing bare Styrofoam.

A note on drying: Slow drying is key!!! If the concrete is allowed to dry too fast it will crack and possibly flake off in the near future. In past articles it was mentioned that cracking could be avoided if you spritzed your drying background with water 3-6 times a day. I will admit that I am too lazy for that, so I just opted to cover the top of my tank with plastic wrap instead. Just put the plastic wrap over the entire tank top, tape it down tight, and then poke some air holes in it to allow moisture to slowly escape. With 6-8 pencil holes it took each of my layers about 4-8 days to dry, which was perfect. Putting you background in a large Tupperwear bin with a lid will also work to slow the drying process. I did this for concrete coats that were applied before siliconing the structure in the tank.

STEP 6: Silicone Your Background to the Glass

The moment of truth!!! Gob on tons of silicone to the back and bottom parts of your background and put it in. There is no turning back now ;) Be sure that your background/tank is 100% dry and clean when you do this step or adhesion will be affected. Glue down any Styrofoam rocks, and glue on any rock overhangs. When things are completely dry you may now want to do some touch ups with the concrete mix.

In my case I did my first 3 coats of concrete on my background (drying it in a large covered Tupperware bin), siliconed it in the tank, and then did the final two finish coats (using plastic over the top to slow the drying). This was a bad idea in the end though because my big 3D overhangs combined with the small tank size resulted in me having to twist my arms in some pretty weird ways to apply concrete in some areas. Of note, I chose not to use concrete stains as they were expensive, and I was not sold on how good they would look besides, algae makes a great natural stain.

Concrete on the bottom: In the end I had some extra mixed cement so I decided to use it to cover the bottom glass of the tank. Why? Well I was planning on keeping shell dwelling species such as multies in my tanks and they like to dig in the sand so much that they often expose glass. I hate seeing glass in my tank (reminds me that it is a fish tank, not part of the lake) so this way they can dig all they want, but all they will expose is concrete.

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My tank before it was filled with water. Note the layer of concrete on the tank bottom. The shadowed lighting effect is achieved with a single desk lamp placed close to the top of the tank.

STEP 7: Rinsing the Concrete With Water

So now you can fill your tank with water. The downside of using concrete is that it naturally buffers of the pH of your tank to levels of pH 9 or 10 over a few days no good for fish. How do we remedy this? The trick now is to circulate the water in the tank for 2-3 days with a power head. Then do a 100% water change and repeat. And then repeat, and repeat, and repeat until the pH in your tank is basically the same as what comes out of the tap after a few days (i.e. until the concrete no longer buffers up the pH). This can take a while, and you may never think your pH will go down but just keep doing 100% water changes until your family and friends confirm that, yes you have gone over the edge with your little fish hobby ;)

Photograph Wood Flooring Chair Hardwood

Water softening salt also speeds up this process so feel free to put in a cup or two with each water change (I personally added about 4 cups to each 15 gallon tank followed by two 100% water changes where I did not add any salt and then I repeated).

Hope you don't mind doing water changes! Notice the amount of overhang, which aids in the final shadowing effect.

STEP 8: Enjoy!!!

When the pH is stable, it is time to cycle and then add fish. Hopefully your long hard work and patience will have paid off. Just be glad for one thing, you now have a truly one of a kind background in your tank!

Water Plant Bedrock Terrestrial plant Grass
Satisfied with my first background on the left, I opted to create a second in my other 15 gallon tank. They now sit side by side on one of my 4ft DIY stands in my living room (call it a 4ft tank with a "free" divider). I also experimented with some plastic plants in the tank fake rocks, why not add fake plants?!

Who would have thought that a couple of lowly 15 gallon tanks, a styrofoam box, and some concrete mix could turn into this?!? Now grab that old 10 or 20 gallon tank and make it look like a 75 gallon!!!
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