Making DIY rock formations with styrofoam sink?

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Towely

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Has anyone accomplished this or attempted it? I've got a 210 gallon tank coming Friday and I want to make some pretty significant rock formations in it. It's a new world cichlid tank so I want a lot of hiding places and separations set up.

Here are my thoughts so far. Obviously start by making the formation with Styrofoam and paint with concrete.

Idea 1) Put many, many coats of concrete on to thicken and strengthen the walls. Then either melt the styrofoam away or with fire. Problems with this are using fire could potentially damage the concrete. Using chemicals could potentially cause water issues(i'm worried even after many, many soaks this could be an issue).

Idea 2) Drill large, long holes all over through the bottom of the formation or top and fill them with concrete to add weight and lessen the amount of styrofoam in the structure. I take it I would have to add a lot of concrete and take a lot of styrofoam out going this route as styrofoam is extremely buoyant.

I don't really want to glue them down as this can be an issue later on when you want to change the tank up and if the glue comes loose that formation is going to shoot to the surface like a rocket.
 

Towely

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There's some good info in there but it doesn't answer my big question. Any way to make these things sink? I do NOT want to glue them to the bottom of the tank.

I was thinking about making small holes at the high points in the decor and then after getting many layers of concrete on pouring acetone(a lot of it) into the base to break all of the styrofoam down and make it much less buoyant. Wondering if anyone has done this before though?
 

fishorama

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I'm not sure where I saw it, maybe loaches.com, but I think he (hx) used the spray expanding foam product to make very realistic tree roots without the floaty/ pH issues of styro & cement. Might work for "rocks" too.
 

Tifftastic

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Would attaching to slate pieces that would rest on the bottom of the tank and could be lifted out be too far away from what you want?
The purpose of building things with styro is that they are lighter than using actual rocks. But if you fill it with cement you're getting similar to the weight of rocks. So, why not use actual rocks and silicone them together?
 

pbeemer

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instead of burning out the styrofoam to get rid of the floatation forces, i would suggest going over it with a hair dryer. this will make the foam shrivel down to ~zero volume without the hazards of high heat and fire. (and burning styrene generates a lot of noxious smoke and residue that you don't want either you of the fish to be breathing.)
 

Towely

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The idea was either to shrivel the foam chemically or with heat completely away OR add enough concrete to the inside to make it just less than neutrally buoyant.

Either way I got a really good deal on some large pieces of driftwood and decided to go that route in the new 210. Eventually I want to make a sand waterfall for my 125 community tank though. Still bouncing ideas around to make that sink. Thanks for the ideas though thus far.

I'm leaning towards just shriveling the foam somehow or scraping as much foam out of the inside as possible(which will still likely leave quite a bit) and attaching a large piece of slate to the bottom.
 
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