Concrete in aquariums?

shrimpyay

Member
I've seen a lot of people on the internet doing various foam-covered-with-cement/concrete backgrounds, and most just mention letting them soak in water for a few weeks to leach out any harmful chemicals. I don't doubt that they DO leach chemicals when curing! But it seems wild that eventually cement/concrete will become totally safe for an aquarium. Am I missing some step, like some kind of sealant to make it safe? Or is it truly just let it cure, then immerse in water for a few weeks and change the water out?

If it's the latter, how long does it need to soak to be safe? How often does the water need to be changed? And will the old water kill a lawn if it gets dumped on the grass? This is such an interesting topic to me.
 

ForceTen

Member
I would not use concrete in an aquarium. Period.
 

John58ford

Member
I do use hydraulic cement in my aquariums. I seal it with pond armor or drylock. Unless you really want lime constantly buffering your ph, and conversely the cement to be slowly eaten by the acid in the water, sealant is important. You can also tint the sealant.

Side note though, hydraulic cement is different than concrete. The hydraulic cement is meant to have constant water exposure, and is very smooth. Concrete is a blend of cement and rocks, it is very chunky. I would believe it would be harder to produce something attractive at aquarium scale using concrete.

For more about my process, I did a write up on one of my tanks. It was meant to be a much larger project but I'm stalled on writing the rest of it, and too lazy to put the hood doors on my stand. The cement starts at post 3 of this thread: John's network tanks part 2 DIY filter and 3D background | 429653 | Freshwater Aquarium Builds
 

MySquishy

Member
You could use it with sealant, like you would in a cement fishpond or fountain, just not plain as it’s porous.

They make products for sealing tile and grout, or granite countertops, some of which may be safe to use once they’ve cured.

[There’s a YouTube video where someone recommended something specific for sealing a fishpond, (and it seems like this person has been doing this for a while) but I can’t find it.]

Unless you already have a cement ornament or something you want to seal so that you can use it, I’d go with what John58ford said.
 

JettsPapa

Member
John58ford said:
Side note though, hydraulic cement is different than concrete. The hydraulic cement is meant to have constant water exposure, and is very smooth. Concrete is a blend of cement and rocks, it is very chunky. I would believe it would be harder to produce something attractive at aquarium scale using concrete.

For more about my process, I did a write up on one of my tanks. It was meant to be a much larger project but I'm stalled on writing the rest of it, and too lazy to put the hood doors on my stand. The cement starts at post 3 of this thread: John's network tanks part 2 DIY filter and 3D background | 429653 | Freshwater Aquarium Builds
It is very possible to create a smooth finish with concrete (but I would still be very hesitant to use it in an aquarium).
 

RayClem

Member
It would be possible to use concrete in a saltwater aquarium or African cichlid tank as the lime leaching from the concrete would not create a big problem. In most freshwater aquariums, it could be disastrous causing issues with pH, hardness, and alkalinity. If you want a textured background, you can use extruded polystyrene insulation board. Owens Corning pink Fomular is one such product. Then paint the background with an aquarium safe spray paint. Although the company does not guarantee it to be aquarium safe, I understand that Krylon Fusion is suitable. Just make sure the paint is thoroughly cured before adding water. The recommend product used to be Krylon Fusion for Plastic, but it seems that product has been discontinued in favor of Krylon Fusion All-in One. I do not know if these products are different or whether the name change was made to indicate the product is suitable for painting materials other than plastic.
 

John58ford

Member
JettsPapa said:
It is very possible to create a smooth finish with concrete (but I would still be very hesitant to use it in an aquarium).
It's not really just the smoothness of a flat surface, the chunk nature of concrete definitely screeds out flat just fine (sidewalks and overpasses). It's the scale of aquarium decorations I don't find it suitable for, the chunks in concrete tend fill in any details you carve out that are smaller than 1/4-1/2 in the first layer, usually full of air pockets as it dries due to the foam core off-gassing/forming bubbles; with hydraulic cement it takes several coats to get up to a consistent layer leaving ~1/8" resolution so details are more clear, and can even be carved into details much smaller if the thickness is not of concern.

A custom concrete blend may allow the level of detail I think lends itself to the small size of an aquarium, or maybe you would like something with less detail like a natural looking artificial fractured slate background. Otherwise I would recommend cement.

I would not recommend either choice unsealed. Although the idea is true that lime added to a cichlid tank may be beneficial, there is no way to control the release of lime in generic cement. Additionally as the lime is used the cement will soften. This process could take a ton of time if you already have hard water, or could be very fast if you have softer acidic water meaning no one person could tell you a custom blend to set your water parameters universally based on experience.

As far as using pond armor, fusion or dry lock directly on the foam; It does work and would be similarly safe. I choose to coat in cement to make the painted surface more durable. It's pretty easy to accidentally gouge into painted foam vs painted cement with basic tools (siphon, tweezers etc). I also have heard of plecos rasping through krylon. For asthetic reasons, I also now dye my final layer of cement just in case the drylock gets rubbed/chewed through, then I don't have a pink or blue (foam core) spot staring at me every day until I rip the tank apart.
 

Lakefish

Member
Speaking from personal experience, I found unsealed concrete was not a problem. I made a three part, very thick (because it set up faster than I thought!) concrete over white styrofoam background for my 72gal. I think I soaked it in vinegar in the bathtub for three or so days before putting it in the tank with magnets. The pH of the water in that tank maintained about 7.4, with the city water being neutral in pH and soft, like rainwater. The concrete did soften a bit over time, but it was in there around ten years, until I had to move house a fourth time to a tiny apartment with no space for that tank. So for the cost, a few dollars in cement, I don’t care that it wouldn’t last forever. If I ever set that tank up again I’ll have fun making a new, better background.
 
  • Thread Starter

shrimpyay

Member
Woah, this got a lot more replies than I expected. I'm not really surprised to hear that it's controversial.

With PondArmor and Drylock, don't those products only come in mostly flat colors? Are you able to color them so that the "rock" you're making look natural? I had heard krylon fusion used to be pretty common, but might be discontinued/rebranded now. Anyone still finding those products?

John58ford, thank you for the in-depth replies! I can see why the hydraulic cement would be the better choice for sculpting detail into a piece at the scale needed for a fish tank.

It totally makes sense to worry about the lime effecting hardness and PH, and given how insanely soft my water is here (2 and 4 degrees KH and GH respectively out of the tap, lol.) it would be a terrible fit for me right now unless I seal it first. I wasn't really planning on building anything of the sort right now, I was mostly just curious to learn about the techniques and materials. You can never learn too much. :)

It's very interesting to hear such a wide range of experiences! I find it fascinating that some people (Lakefish, thank you for sharing your experience!) were able to keep it unsealed in the aquarium for many years without any detrimental effects.
 

John58ford

Member
Lakefish said:
The concrete did soften a bit over time, but it was in there around ten years
Awesome to hear you had luck with one unsealed, and 10 years is definitely a good long time. You said it was pretty thick, how thick was the concrete? I am wondering if thinner might have gotten soft enough to crumble faster.

shrimpyay said:
don't those products only come in mostly flat colors? Are you able to color them so that the "rock" you're making look natural?
The colors are in fact pretty lame. I use mostly drylock, and it comes in a goopy white. I use the quickcrete brand dye, I have seen it in black, red, yellow and a greenish blue. You can mix it to make most darker colors that you would find on a generic rock formation, but it would be hard to make more elaborate stuff like turquoise, or paint a mini figure if that's your kinda thing.
 
  • Thread Starter

shrimpyay

Member
John58ford said:
The colors are in fact pretty lame. I use mostly drylock, and it comes in a goopy white. I use the quickcrete brand dye, I have seen it in black, red, yellow and a greenish blue. You can mix it to make most darker colors that you would find on a generic rock formation, but it would be hard to make more elaborate stuff like turquoise, or paint a mini figure if that's your kinda thing.
Do you mix the quickrete dye into the drylock? If so, I'm sure I could make that work fine mixing into white. More bright colors would be nice, but yeah, I can't imagine you'd be able to do much with those. Thanks for the tip!
 

John58ford

Member
shrimpyay said:
Do you mix the quickrete dye into the drylock? If so, I'm sure I could make that work fine mixing into white. More bright colors would be nice, but yeah, I can't imagine you'd be able to do much with those. Thanks for the tip!
Yes I do mix it straight in. To be honest doing my basalt colored project it looked very good highlighted (I mixed the black into the dry lock, then started diluting it with white and adding some of the green color to highlight cracks), until I filled it with water and turned on a full spectrum light. Then it looked like the Swiss alps. You should play with your color pallet on a test chunk and submerge and light it before you decide to paint a large piece. Since it's truly 3d you may not have to do as much coloring as you thought you would.
 

Lakefish

Member
John58ford said:
Awesome to hear you had luck with one unsealed, and 10 years is definitely a good long time. You said it was pretty thick, how thick was the concrete? I am wondering if thinner might have gotten soft enough to crumble faster.
I believe it was between half an inch and two inches thick. Lol. I "outsmarted" myself and used quick-setting post hole mix. Boy, that was dumb. But several hours to set was waaayyyy more than my patience could handle
 
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