# How to elevate rocks, substrate



## somewhatshocked (Aug 8, 2011)

Egg crate is definitely an easy solution. 

I wouldn't use expanding foam, as it will ultimately float. Just build some egg crate 'cages' with zip ties that you cover in something like fiberglass window screen (will help prevent substrate from sinking down into the cage) - or fill the inside the cage with lava rock - and place the large rock on top of it. 

Some people also put lava rock in mesh bags and use that to build the substrate/hardscape up.

Those are the two easiest, cheapest ways to do it. You won't even need something like foam. And if you need to stick rock together? You can use aquarium-safe silicone or some reef epoxy putty.


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## DaveKS (Apr 2, 2019)

You might find this guys technique for doing his waterfall useful. He's using expanded PVC it looks like but you can do same thing with coroplast by folding/building/glueing boxes and then fill with lava rock and put some fiberglass window screen on top, making substrate retaining walls, glueing some rocks into place. Same techniques can be applied to what your trying to do.


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## mossman77 (Aug 5, 2019)

somewhatshocked said:


> Egg crate is definitely an easy solution.
> 
> I wouldn't use expanding foam, as it will ultimately float. Just build some egg crate 'cages' with zip ties that you cover in something like fiberglass window screen (will help prevent substrate from sinking down into the cage) - or fill the inside the cage with lava rock - and place the large rock on top of it.
> 
> ...



What about stagnate water, waste, decomposing plant matter, etc getting trapped under the structure? This was why I was thinking foam would be best. I think it would have a hard time floating with a heavy rock on top of it.



DaveKS said:


> You might find this guys technique for doing his waterfall useful. He's using expanded PVC it looks like but you can do same thing with coroplast by folding/building/glueing boxes and then fill with lava rock and put some fiberglass window screen on top, making substrate retaining walls, glueing some rocks into place. Same techniques can be applied to what your trying to do.
> 
> https://www.youtube.com/watch?v=2IfvTTXr9yE



Wow. That was pretty sweet! I'm looking to do something similar, but no waterfall, more water, and fish.


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## DaveKS (Apr 2, 2019)

With proper circulation in tank, micro currents will carry oxygen down to those levels so bacteria will take care of any minuscule particles of detritus that do make it down there through top substrate layer.

Also not a fan of though of putting a substance in a tank the WHO classifies as a carcinogen.

https://uspirg.org/news/usp/world-h...increased-cancer-risk-chemical-found-plastics


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## somewhatshocked (Aug 8, 2011)

Like @DaveKS said, it's not an issue.

Water flows throughout a tank - even in the substrate (as long as it's not compacted) in a normal tank. That's why people use egg crate to build structures like that - _because_ they allow for flow. It's just a false bottom like is used in a vivarium.



mossman77 said:


> What about stagnate water, waste, decomposing plant matter, etc getting trapped under the structure? This was why I was thinking foam would be best. I think it would have a hard time floating with a heavy rock on top of it.


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## DaveKS (Apr 2, 2019)

And if your a clever builder you can implement a low flow reverse flow system that actually feeds oxygenated water down there to keep lava rock working as a supercharged bio filter and will push nutrient/oxygen laden water back up through substrate right to plants roots and CEC binding sites. Plants love it.


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