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Making a large rock semi-buoyant

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OK, stupid question time...

I going to spray a bunch of Great stuff foam and set a large 10# rock on top of it. So when the tank is filled with water, that should reduce the load on the bottom of the tank ie: the effective weight of the rock may be 7#. Of course too much foam and the rock will now float. 

This is for my Mbuna tank I am building (75 gallon), the entire bottom will be large rocks. It will be a total of 100-150# of rocks. 

Am I missing something here? 

Here is my work in progress



Edited by madmark285
added stuff
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not exactly sure that it's necessary, the weight of water in a 75 is roughly 630 lbs. believe standard capacity is over 10 lbs per gallon. you can by what's called egg crate, which will lesson point of contact weight (rock to glass) and more evenly distribute it. but the weight should be no problem as is. the other thing is, not sure great stuff is actually fish safe over long periods

don't know if you've seen it but John from KGTropicals (youtube ) has a great looking yellow lab tank completely made of stone in the back

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On 2/2/2024 at 12:34 PM, Tony s said:

not exactly sure that it's necessary

Much more complicated. Under the rocks will be caves (I am calling them condo's) built using PVC material for the Mbuna. The rocks will be held up by a bed of foam (a combo of great stuff and 3# density polyurethane foam). The goal is to get 22-24 condo's in the tank.

The pourable foam will also create a slope effect. So adding buoyancy is not the main objective, just a side effect.  

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On 2/2/2024 at 12:49 PM, Tony s said:

you and John think alike, I think. 

Yes we do and thanks so much for that link! Finally someone else gets it, make a Lake Malawi tank ie: all rocks!  I also have the same problem as John, finding rocks and I need more rocks! I did find a bunch of black rocks, may go back where i got them for more. I did start a journal last year, here the link:  Mbuna Condo

Below is the look I am going for:


So the bottom layer of rocks are set in a layer of foam. 

This is version 1 of the PVC condo structure, I am currently working on version 2. 


So concept here is, stuff a balloon filled with sand in all the condo then stack a layer of the base rocks. Then pour foam on the whole frame. Then of course remove the balloons. What John says is so true, trying to find the right combo of rocks stacked to make a cave is very difficult. So I am cheating....

On 2/2/2024 at 1:34 PM, Tony s said:

great stuff makes a product called pond and stone.

Thanks for the tip! I plan on using Great stuff foam as a base filler then 2 parts pourable foam (3# density)  I got from US Composite. All the foam will be coated with white silicone caulk covered with aragonite substrate. The US Composite foam is for marine application. 

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I used great stuff black for my hollow island for my gobies. Great stuff pond and stone is a ripoff. It’s $4 more just because it’s marketed towards aquarists.


Great stuff has been used to fill out zoo and aquarium scapes for decades it is non toxic and doesn’t break down in water. It’s hydrophobic as it cures as well.


I made a ‘Z’ out of PVC black 3” dia pipe then layed 1 layer of foam over it and started setting the stones. Then once the great stuff cures I use a razor blade to carve it to match the rest of the stone. After a year it’s completely unrecognizable from other stone as algae has taken hold. The whole island weighs about 20#






Edited by Biotope Biologist
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Thanks! Just couldn’t find any information on the longevity of it. Or more importantly, the fish safety. Only just now hearing of the stone and pond product. Have only been using it great stuff since it was made. Just on doors, windows, livestock buildings 😀

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