# Idea for DIY rock and styro background in filled tank



## kriskm (Dec 1, 2009)

Just asking some very preliminary questions. I would like to make a styrofoam background for my 6' 120 gallon acrylic tank, but I don't want to break the tank down to attach the BG. I have some large, flat paving stones, so my thought was to buy some sheets of acrylic (likely 4, for 4 sections of BG), and attach a large rock to the bottom of each one to anchor it, then fill in with styrofoam. I don't plan on the BG being very deep, just enough to cover returns and heaters and such. I'm concerned about how I can attach the rock and the styrofoam securely to the acrylic sheets, since I've read that silicone doesn't stick so well to it. I'm considering Gorilla glue (though it's gotten mixed reviews in posts here), or this stuff:

http://www.homedepot.com/h_d1/N-5yc...splay?langId=-1&storeId=10051&catalogId=10053

I plan to paint the stryofoam with Drylok and just set each piece in the tank against the back of the tank.

Any thoughts?


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## mightyevil (Oct 23, 2008)

To be honest with you, I don't think you will be able to keep any styrofoam down with anything unless you silicone it to the bottom or back panel.

I have used silicone to attach backgrounds to acrylic tanks before and I have never had a problem with it.


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## kriskm (Dec 1, 2009)

> I don't think you will be able to keep any styrofoam down with anything unless you silicone it to the bottom or back panel.


I will need to weigh the rocks I use. As long as they weigh more than the water displaced by the styrofoam and rock, and everything is well attached, I should be good to go.


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## codenametorch (Dec 27, 2009)

kriskm said:


> > I don't think you will be able to keep any styrofoam down with anything unless you silicone it to the bottom or back panel.
> 
> 
> I will need to weigh the rocks I use. As long as they weigh more than the water displaced by the styrofoam and rock, and everything is well attached, I should be good to go.


I think you still might want to consider draining the tank and permanently attaching the foam to it. Styro parts shoot out of a tank like a rocket if they become dislodged.


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## kriskm (Dec 1, 2009)

Sounds like I can securely attach the styrofoam to the acrylic with silicone. How about attaching the rock to the acrylic? Would marine/aquarium-safe two-part epoxy (like the PC-11 in the link) be a good bet? I like the fact that it can fill gaps. I will make sure that each section (of rock and styrofoam on acrylic) sinks before placing it in the tank.


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## mightyevil (Oct 23, 2008)

Attaching the rock to the acrylic may work but you gotta remember that silicone does not stick too well to some rocks while it sticks perfectly to others. It is going to be very tricky trying to do anything with the tank wet. Moreover, I think it would be quicker to put your fish in a bucket or tub and put your filters in there as well than to try to work on the tank while it has water in it. just an opinion :wink:


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## kriskm (Dec 1, 2009)

> Attaching the rock to the acrylic may work but you gotta remember that silicone does not stick too well to some rocks while it sticks perfectly to others.


Exactly, that's why I'm not planning to use silicone for the rocks, but wondering if the epoxy would work instead.



> It is going to be very tricky trying to do anything with the tank wet.


I don't plan to work inside the wet tank at all. I will make the background in sections outside the tank on nice dry land. I want to make it in four pieces, so I will start with four sheets of acrylic, each about 1/4 the size of my tank, or a little smaller. I'll attach a big rock and styrofoam to each sheet of acrylic, carve the styrofoam on each one, seal it with drylok, and let it completely cure outside the tank. I will make sure all the pieces sink (in a deep bath tub?), and then place them in the tank once they are all finished by just leaning them against the back. Hope this makes sense.


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## Malawi_Junkie (Nov 26, 2009)

Might work.
My concern would be that a rock heavy enough to hold down the styro could be a tank buster if things go wrong. I recently had a BG seperate and shoot out of the tank which scarred the sh_t outta me, I greatly under-estimated the styro.
IMO draining the tank and silicone permanently to back would be the best bet. It would be cheaper and faster.
Then again I am curious if it would work.


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## mightyevil (Oct 23, 2008)

Well, I can tell you this! I made a background and used magnets to hold it against the back panel, the rock was wedged from the bottom of the tank to the top and the sand definitely helped keep the background in place as well. It worked very well, so much so that I had trouble tugging on the rock to come off when I decided to take it off. Here are some pictures of that...

behind the background I installed magnets, that is what those black dots are...









This is what the tank looked like from the back with some of the magnets...









and this is what it looked like...









Here is the thread if you are interested in this method...
http://www.cichlid-forum.com/phpBB/viewtopic.php?t=198716&highlight=


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## sweety (Jan 10, 2010)

I'm thinking of making a similar BG but don't want to perm fix it as the tank may get used for something else in time :roll: I was wondering if at the bottom of the BG if the sand & a couple of rocks would keep it in place so it didn't move forwards towards the front of the tank & if I fitted a couple of hooks to attached to the back of the styrofoam which were U shaped to go over the rear glass I then could tension these with cable ties down to a fixing on the tank stand.
Not sure if it'll work but it was an idea which I thought might help :roll:


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## mightyevil (Oct 23, 2008)

I think that the best way is the wedge the background between the bottom panel and the tank's rim. It worked for me but it may not work for others, like I said before the magnets and sand helped significantly but all the techniques must work together in order to keep the stress off of one or two particular areas. Make sense?


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## sweety (Jan 10, 2010)

Yes I see where your coming from & love the idea of using magnets. I sadly don't have a top rim on the tank i'm going to use thats why I sort of came up with the plan I did.


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## mightyevil (Oct 23, 2008)

Is your tank glass or acrylic?


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## alfadog (Dec 18, 2009)

dude! thaT Looks awesome, nice job :thumb: :thumb: :thumb: :thumb: :dancing: =D>


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## kriskm (Dec 1, 2009)

Nicely done BG mightyevil. How spendy were those magnets? My way, with the acrylic, big flat rocks, epoxy to attach rocks, styrofoam, drylok, cement tint for drylok, silicone, is not looking very cheap.



> I was wondering if at the bottom of the BG if the sand & a couple of rocks would keep it in place


I looked into the specific gravity (density compared to water) of rocks, and it looks like most fall in the 2.5 to 3 range. This means that a rock would have to displace more than 2.5 times its volume in water to float. With rock attached to the bottom of each section, you could have 1.5 times the volume of the rock in styrofoam above it, and the whole thing should still sink. With the rock at the bottom and styrofoam at the top, it should also stay upright. With an acrylic tank, I have a nice rim to shove things under, but really, it shouldn't take much to keep it there.


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## mightyevil (Oct 23, 2008)

Thanks guys, that background was taken off already...I have a thread on the newer one under "my next DIY background" and I'm told that it is better.

I would hate for you to underestimate the buoyancy of styrofoam and have something unexpected happen. I would advise on some testing first.


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## JWerner2 (Jul 7, 2008)

I would still secure it with the magnets or something.

I thought I could take short cuts plenty of times by wedging it under the rim. All in all it never worked.

How about the stuff they use for gluing reef rock together?

Does that only bond in salt water or could that bond in fresh water? I used it a few times for frags back in the day and its 100% safe for use in established tanks.


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## mightyevil (Oct 23, 2008)

I dont think he wants any kind of adhesive on the tank.

Kriskm, if you are on a budget then the magnets may be out of the question. I used epoxy coated magnets from magnet4less.com and I got 60+ magnets, they cost me like $70-$80. Of coarse you can get some cheaper ones but you need something with at least 27 lbs of pull strength or more and use them in pairs like I did to double the strength.


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## sweety (Jan 10, 2010)

mightyevil said:


> Is your tank glass or acrylic?


Mines glass so I suppose I could add a rim to help it stop in place. TBH hadn't thought about it until you asked what the tank was made from


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## mightyevil (Oct 23, 2008)

sweety said:


> mightyevil said:
> 
> 
> > Is your tank glass or acrylic?
> ...


you can probably use a 2" piece of glass cut to the length of the tank and use that as a stopper. You would have to use glass the same thickness as the glass of the tank. This would add additional strength to the tank, not much but some. :thumb:


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