How to make an amazing rock-wall background 101

iZaO Jnr
  • #1
Hey guys!

Been a while since i've posted. Been super busy. Nevertheless I'll show you what i've been up to and show you how easy it is to do.

So many people, including myself asked the question countless times and never found a solid answer, or a simple one for that. So instead I just went on instincts and did it without the help I wanted. Building a rock-wall background is actually so easy, a 10 year old could do it. So it could even be a fun project for the younger members. I know it was for me.

The way to go about building this rock wall from scratch is simple, and there are many ways. I'll show you mine, but mention other methods as well. Remember the idea here is to make it realistic. What's the point of a rock wall that looks like a dam wall in your aquarium right?

Before I start, a couple of things I found when building my first 2 backgrounds.

1. Keep it simple. Simpler backgrounds can actually look better in a tank. Why? Because a simpler background is easier to detail and finish. Once you get the hang of building them, then you may feel confident enough to build a more complicated one.

2. Know your plan before you start. My first background I just went off and did what I felt was right. It turned out okay, but not great. When I made just a basic sketch of what I wanted for the rest of the ones i've done, then they turned out quite superb.

3. You need some basic common sense. Think every step through before you do it. For example, my first background, involved a lot of siliconing small pieces onto the face of the background. What I didnt think of is silicon doesn't melt, it burns, and hence left terrible marks on the background. My solution to this in my next background was to silicone bigger pieces and melt them to get them smaller.

Okay, so here goes.

Material List:

- Sheets of polystyrene. Thicknesses vary here so consider the following. A thinner sheet will mean you can't melt as much, a thicker one will occupy a lot of volume. I always go for 1 1/2 inch sheets so that I can melt what I want and don't sacrifice too much volume.

- Butane/Propane torch - This will be used to melt the polystyrene to make it look natural. I also used it to fade pieces into one another to create a bit of undulation.

- Aquarium Safe Silicon - I used one large tube to do an entire background measuring 5' x 2.5'. You will need another to silicone the background onto the tank.

- 2 Containers - This will be to mix the concrete covering, 1 for the concrete, another just for water.

- 1 spray bottle - Concrete works much better when it is wet. It also dries up to three times as strong with frequent curing (simple spraying twice a day will make it invincible).

- 3 or more Paint brushes. Just buy the cheap, rubbish ones. You aren't looking for a smooth bristle or anything so save yourself the money. Get varying sizes so that you can cover large pieces at a time, but also detail corners and cover small gaps.

- Additional pieces of polytyrene. These will be used to simply create a bit of undulation and other cool stuff. You can use anything here, provided you can silicone it to the large sheets.

The first step to do, is to cut and silicon any large pieces together that will fit the back of your tank. Again, think carefully about this. My tanks have cross braces across the top edges, both length and breadth ways. So I had to do mine to 2 pieces.

Once you have the silicon base together and dried (always give 48 hours for silicone to dry), the base should be pretty strong. In fact, the joints you make should be stronger than the sheeting. If you need, reinforce the joints with extra sheets.

Once you have the base, you can start getting the smaller pieces and offcuts onto the base. Cut them if you need, and shape them to the general shape you want. Remember here the idea is to smooth out the shape when you're melting. So you need to cut the piece to what you roughly want it to be before just getting it straight onto the base. Once these are all onto the base, you should start to see how the whole thing will look.

The next step is my favourite. Melting the polystyrene. Before I start, please make sure you are in a well ventilated area for this. If there is very little or no wind, then rather do it outside. This can have devastating effects on you if you aren't in a properly ventilated area. You do need a torch here. Lighters or similar don't work, believe me. The way to go about melting, and this is very important, is to remember you are making it all fade into itself and smoothing out rough, unnatural edges. You ARE NOT changing the shape of the background with the torch. You will get a feel for the melting. I would suggest just practicing a bit on a off-cut piece or something, just so you can get the idea of how to do it.

The best way to do the melting is a sort of "air-brush" method. Left to right and back, slowly and carefully. The slower you melt the polystyrene the better it will come out. Tilt the torch as necessary to get to edges and corners. Just continuously keep an eye on how the polystyrene is reacting. If you see it starting to melt away quickly, pull the torch away immediately and blow on the area that is melting. Believe me you can very easily blow a gaping hole into the sheet if you aren't careful. Another thing to watch out for is the silicon. Plan this ahead of time and make sure that the silicon is nowhere near where you will melt, and melt nowhere near where the silicon is. Silicone catches alight when heated and it doesn't go out easily. So this is the one thing to avoid most, as it can be the one thing to completely ruin your background. Always keep the face you are melting weighted down because the action of melting tends to have a warping effect on the sheets.

Once the melting is complete, I would recommend going over the entire background once more, very lightly. The melting also hardens up the polystyrene so it will only serve to help you. Most importantly, make sure that edges and gaps are done as well as you can. It is inevitable that a few gaps will form because of the melting. Gaps are easy to solve provided they are not big enough to get your thumb into them.

Once your polystyrene is melted well and you like the look of the entire background, then you can start with the next step, the covering with plaster.

How to mix the concrete covering:
- 1 part cement
- 3 parts plaster sand (MUST be plaster sand, or finer)

It's that simple. When adding the water to this mix, mix the contents very well while the water is being added. The amount of water will depend on many things. The consistency you are looking for is like a very sloppy bowl of oats. Almost like a liquid paste. Mayonnaise is a good example.

Now when it comes to the covering, you have to consider of course the colour and tone of the background. The way to adapt this is with the use of oxides, and differing cements.

Cements come in black, regular and white. Black will give a very dark colour regardless of what you add to it. Regular will also be very dark, but oxides will have the ability to adapt it better, and white will leave you with a very light colour.

Oxides come in an infinite amount of colours as I can tell and are quite easy to use. For a strong effect of the oxide, use 1 part oxide to every 10 parts of cement. For a lighter hue, use 1:15, and for a very subtle hue use 1:20. I generally use 1:20 because the colour must be subtle as it is in nature.

On a side note, once submerged, the water darkens the colour quite a few shades of your background regardless, so always account for this in your plan.

Once the covering has been mixed, apply it to the polystyrene in a thick layer, all over. You don't need to focus on the gaps and details too much yet, we will get there. The idea here is to just coat the polystyrene to create a seal with the concrete. This will prolong the life of the background. When spreading the concrete, you will need a container of water to dip your paintbrush into. When it gets difficult to spread the concrete, just dip your paintbrush and continue spreading. You will realise how easy it makes the spreaing.

Once a decent coat has been applied, repeat another 3 times at least. This may seem a bit much, but trust me, it is needed. You need your background to be strong and well sealed. Cover all faces of the background and make sure that very little or no polystyrene is exposed to the water.

Now that this is done, your background would've increased its weight tenfold. This is a good thing, but just be careful when handling it from now on. Now you can start covering the gaps and crevices. Use a thicker paste to do this and just fill it as you see necessary. There is no specific method to this, you just have to simply get the concrete to fill and smooth over the gaps.

Once you are happy with the coating. It is then curing time. Concrete is generally dry in 5 days, but by curing you delay that process, which is a good thing. Give the entire background a very wet spray twice a day. You will see how quickly the concrete absorbs the water. In 14 - 18 days your background should be dry. I recommend going to a month. This also ensures that the concrete will leech less lime into your water anyway, which is important. Once your background is well dried, you can then silicone it onto your tank. Dont use too much silicone but don't be stingy either. Apply beads every 3-5 inches and make sure you get enough in the corners as well. You need to make sure that it fits well and doesn't have gaps at the back. Gaps that fish can get into will wreak havoc. If there are small entry ways and gaps, you will need to fill them with either concrete or silicon. I use both.

Once onto the back of your tank, make sure you weight the background well to keep the edges, corners and middle down well. Leave for 48 hours to dry.

Once this is done, you aren't finished yet. The last step is to cure and condition the concrete inside the tank. Fill your tank with water only. Circulate the tank with some pumps or airstones or whatever. Do this for 3 days. Drain the tank, and repeat twice more. You can add ammonia to the water if you like to get some immediate algal growth on the background, but I do recommend adding potassium salts. 1 kilogram per 20 gallons of water should do fine.

The reason you add this is to lower the pH of the water severely. This action will cause the concrete to leech lime very quickly, because the natural pH of concrete is 11. By forcing the concrete to leech the lime very fast, it will ensure that by the time you finish this step, the concrete's leeching will be insignificant, making it safe for fish.

There is one way to prevent you having to do this step, or if you don't like the idea of lime leeching into your water. An epoxy resin can be used to seal off the concrete. Make sure you get a 2 part epoxy resin that is used for sealing ponds or potable tanks. DO NOT use the epoxy for flooring, as this sometimes has very harmful compounds. Epoxy is completely optional on your background, and personally there is only 2 conditions I would recommend using it.

1. When keeping super-soft water fish
2. When using this background in a saltwater system.

And so that concludes how to build a rock wall background. Here are some pictures to help you understand the steps I explained. Apologies for the lack of good ones, as I wasn't very photo happy when making these.


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iZaO Jnr
  • Thread Starter
  • #2

  • #3
Very nice!
been looking for some ideas for my new 6footer, my old method (see signature) wasn't really viable due to the sheer amount of rock required but I may try to incorporate some natural rock into this method.
  • #4
Great project, and thanks for the clear explanations, been wanting to try one of these and you make it sound very doable; one question though... what type of torch were you using to melt the pieces with?
  • #5
Great post iZaO Jnr! It's always fun doing DIY projects like this. I made a live rock wall for saltwater aquariums following a very different process. Good stuff and thanks for sharing.
iZaO Jnr
  • Thread Starter
  • #6
Very nice!
been looking for some ideas for my new 6footer, my old method (see signature) wasn't really viable due to the sheer amount of rock required but I may try to incorporate some natural rock into this method.

Your previous method was one way I have tried before but it really ended up being to complicated and fragile. You could very easily incorporate some natural rock into this though, provided it wasn't too heavy. If you do have a go at this let me know how it goes

Great project, and thanks for the clear explanations, been wanting to try one of these and you make it sound very doable; one question though... what type of torch were you using to melt the pieces with?

No problem, glad to have helped. The torch I used was a small butane torch


Any torch similar to this should work.

Great post iZaO Jnr! It's always fun doing DIY projects like this. I made a live rock wall for saltwater aquariums following a very different process. Good stuff and thanks for sharing.

Thanks very much!... It took a while for this project to come well but a bit of practice and it actually gets really quick and easy.

I remember that thread well. I absolutely love that project, and will be a definite for when my salty gets up and running.

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