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I'm thinking of doing a DIY foam and cement BG but the Buoyancy freaks me out a bit. I'm just picturing my silicone job failing and this thing comes shooting out, snapping the cross bars etc.

Since all I want is a rocky effect, no dramatic ledges, caves etc. , Basically I just want a 3d looking back drop, then I wonder what's to stop me from building a form and pouring the entire thing out of concrete?
 

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Nothing technically could try it depends what size you are talking about

Problems could see,to heavy to move,To thin might break into little pieces during transfer

About it really
 

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I am planning on doing just that as well. I plan on carving out the styrofoam and using it as a mold but I am going to make a light weight concrete just heavy eough to sink out of portland cement type II, perlite, sand, and a little shredded styrofoam. I also am going to lay fiberglass mesh down for added support (tape for repairing drywall) in the middle of pouring. to prevent it from sticking I'm going to coat the foam with cooking oil. I'm also going to make caves using foam. Check out my 200 gallon tank build. I'm not that far yet.
 

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I did something similar to what you are thinking but I used egg crate or lighting grid for drop ceilings. I never took any pictures during this project because it's fairly fast paced and I didn't want the cement to set up too fast.

I cut the egg crate to fit the dimensions for my tank but made the panels small enough to fit between the top braces of the tank.

I set up a pair of sawhorses in my garage with a piece of plywood for a table. Cover the plywood with a piece of visquine or trash bag. Set the egg crate flat on the table.

Lay out the rocks to cover the egg crate in a natural pattern. It's helpful if some rocks overlap where the seam will be between panels so it's not as obvious when installed in the aquarium.
When you have the rocks where you want them, take a picture because you will need to remove them for the next step. Just remember to not place any rocks overlapping the bottom edge of the egg crate or the side that will be against the glass side panel of the tank.

I used Quikrete brand Quick Setting Cement or Quikwall Surface Bonding Cement (fiberglass reinforced) and mixed it according to the directions. I added cement colorant that was similar to the rocks I was using.

I completely covered the entire surface and all the pockets of the egg crate with the cement and then added an additional 1/2" of cement above the surface to embed the rocks in the wet cement.

You can use additional cement to 'grout' around the individual rocks to insure they are securely attached. I even sprinkled sand on the wet cement to make it look more natural.

Let everything cure according to the cement directions. I misted water over the entire surface to slow the curing process and help eliminate cracking. After a few days, I soaked the individual rock 'panels' in fresh water, changed daily, to help reduce the alkalinity.

When I was ready to put the rock panels in the tank, I used thin foam spacers between the bottom of the tank & the bottom of the rock panel. This helped to prevent any pressure points from contacting the glass. It also made it easier to slide the panel into place.

I only had to silicone the top edge of the rock panel to the aquarium to hold it in place, using a couple clamps until the silicone set up in 48 hrs.

Here is a couple pics of the rock panels installed in the 125G tank.

Left side



Right side



Filled with water, driftwood and live plants



This is the second aquarium I've used this process in. The only disadvantages with using real rocks and cement is that the background is pretty heavy. I found that making the individual panels smaller makes installing them much easier.

So for my 125G tank that has two center braces, I made 4 separate rock panels. It was much easier to install them.

Sorry my post was so long but I hope it gives you some ideas on how easy it really is to do. You could probably use the same process but make your rocks out of Styrofoam and attach them the same way. Using eggcrate filled with cement may just counteract the buoyancy of the foam.
 

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Make the structure neutral buoyancy, leaving much of the foam at the top portion. I had my 3-D background constructed with steel reinforcement cast in concrete, giving skeleton to the structure. Surface concrete is very thick throughout. Patience is the key.
No silicone is required. Installation of the sectional background into position is by wedging. No worry of indoor mini tsunami!

This is my DIY which i did last year.
 

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WOW!! Beautiful job venustus 2008. Very detailed and excellent step by step instructions. It puts my lame description of my background to shame.
 
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