Filling behind 3D Aquarium background - expanding foam?

I think that the regular expanding foam is OK as long as it doesn't have chemicals in it (like the Great Stuff Pestblocker). Lots of people make backgrounds out of it and it turns out OK. Some people have had problems with the regular yellowish foam coming apart from exposure to light over time. There are ways to prevent that (for example, paint with Drylok, cover with epoxy, etc.) or use the black pond foam.

Bottom line, I think you'll be OK using it for what you are doing. I'm doing the same thing for my background that I'm building. I carved foam board and glued it together to get the effect that I wanted, but it leaves space between it and the board that I'll attach to the glass, and I'm filling it with Great Stuff (regular yellowish foam). Part of my background that will be exposed to light will be made entirely of the black pond foam, but I'm painting it with Drylok too (not worried about it coming apart, but painting it for cosmetic purposes). Also, beware, because it's very buoyant, so you'll have to secure it to the tank. Are you planning to spray it to the glass? If so, it makes a good bond, but is very difficult to remove if you ever want to change your background. I personally and making mine permanent, so I'm using it as glue also.

To help counter buoyancy issues, I'll be filling the gaps with a combination of river cobble stones and the foam.

Here's my work so far:


Here's a side view where you can see where I'll be filling in the gaps that will be filled with foam, although I've since revised the base:

Just to follow up. I filled up the gap behind the carved foam with river stones from HD.


Yeah, I know, I bought some rocks, but at the time the weather was terrible and these were cheap, and they were there . After arranging the rocks, I filled in the gaps with Great Stuff:


I did this for all three sections of my background. There was quite a bit of work after that, but here is what it looks like as of today:


Next steps: painting with Drylok, building and artificial root structure, installation into the tank, then tank set up.

I thought that maybe if anyone was interested how the wall turned out, and the roots, that I'd share where I am so far. I wanted to follow up with my progress, but also to keep everyone updated once I fill the tank with how my plan to keep it all from floating works out. I painted the wall with Drylok mixed with cement dye in various shades, and installed the three sections into the tank with silicone, lots of it. I also created faux roots out of PVC pipe, rope, plaster wrap, spray foam, foam board, wire, plastic tubing, and grout, then painted them with Drylok mixed with cement dye. There are real river rocks inside the wall structure, as I showed in my previous post, and all three pieces fit tightly into the tank (top to bottom, and together), and are attached very firmly with silicone. The faux roots stand alone, and the grout and plaster wrap should be enough to weigh them down, as there isn't much foam involved with them. They are very heavy. If it turns out that they float, then I will figure out a way to weigh them down. I want them removable so I can get to the compartment to clean the canister filter intake that is inside the section of wall that juts out behind the roots. The purpose of the roots is to simulate an overhanging creek ledge, since this is a native stream tank. It will hide a strong powerhead from the front view of the tank. Current will also be generated above the roots on the left side of the tank by the canister filter output. I plan on keeping river stargrass on the right side of the tank, planted in river gravel, rocks and sand as you'd find naturally in a stream. I purchased a Fluval planted 3.0 light and need to purchase a glass top. I also collected river rocks of various sizes, washed and scrubbed them, and need to scape the tank with them. Once I have the rocks and sand in place (this weekend, I hope), I will fill the tank and cycle it. That is the big test, to see if what I posted in the previous post works, along with the tight fit and tons of silicone holding the wall in place, to see if the wall stays put and the roots don't float. Here are pics so far:

FTS:


From the left side:


From the right side:


After installing everything, I got to thinking that I could damage the foam wall during cleaning if I wasn't careful, so maybe I could coat it with something. After some research, I decided on buying some Polygem clear coat epoxy. I applied it on the wall and the roots. It really firmed everything up, and I feel good about the clear coat protecting my work. Supposedly, when under water, the shiny look disappears. We will see when I fill the tank. I hope so, because I really don't like the shiny look. This Polygem stuff is used by museums and public aquariums, so maybe everything will be OK. Even it the shiny look is still a little bit there, maybe when some life grows on the rocks and root surfaces, things will dull again. Here's an FTS pic after the clear coat:


My next post will be the results of the test, filling the tank, seeing if the rocks and/or roots stay put or float.

Thanks for looking.

I made progress, but... A couple days ago, I remembered that there is a stream close to my house where I could drive up and park right next to the water. So, I went and checked it out, and sure enough, there was a gravel/sand bar that looked great. The colors and size were perfect. Then, yesterday, I screened, washed and sifted the gravel. Here's a tip though. When you have to carry a 5 gallon bucket worth of wet gravel or rocks to your house from the vehicle, and then downstairs, it's much easier to divide it into two 5 gallon buckets and carry it in balanced. A month ago, when I collected the rocks, I carried the full 5 gallon bucket down and it wasn't fun. Also, washing creek gravel took a lot more time than washing store bought gravel. It seemed like it took forever. I think that I spent a good 3 hours or more washing it. Anyway, here it is divided into the two buckets:


Then, I took the rocks out of the bucket and sorted them out by size and shape, and set aside the ones that I wanted to be seen the most and arranged them on my workbench (which is the stand that I built for my 100g oyster reef tank).


After that, I went ahead and rockscaped the tank, then filled it with water. This was a test run, to see if the wall held up and the roots didn't float. I also set up the powerhead and checked out the current. It ran perfectly. I didn't use this design to achieve unidirectional flow. Rather this current, along with the spray bar, should simulate a stream side eddy and undercut bank. I didn't take a picture because the water was still cloudy, and, I didn't have my canister filter set up. I will have that done tonight.

I also received the light that I ordered, a Fluval Planted 3.0. I'm really happy with it. I plugged it in, downloaded the phone app, and configured and previewed it. It's really cool. And, the best part is that, remember the shiny look of the epoxy after it finished drying, and I was worried about it? Well, what they advertise is true. It really does disappear underwater. Everything should look great when I'm finally done.

However, and now the but... I noticed that the roots stuck out of the water about 1/4", which is no big deal, but, it would have affected my ability to run the spray bar and shoot water over them to simulate the stream. I thought that maybe, when scaping the sand and rocks, that I just needed to adjust it, so, I pulled out the roots, moved the sand and rocks to the right side of the tank, and stuck the roots back in. And...they float. It isn't bad, but, it's enough to mess with my design concept. So, I unplugged everything and will work on fixing that asap. I pulled the roots out and set them in a container to drain them as water does get inside them. I'm not too worried about that because everything is inert that I used, and it enters very slowly. Although, I'm considering another coat of epoxy on areas where water might seep in, just in case.

The roots float at the top of the fixture, moving it away from the wall and up over the water surface. The bottom of the roots seems to stay down, somewhat because of sand and rocks, but, more because there was less foam used there. So, I think that I can leave the bottom alone, and design some sort of hook and latch system at the top, in the back, out of sight, that will hold the roots down.

Next steps include fixing the roots, draining the tank, scooping out the rocks, sand and gravel, add sand and gravel again, rescape the tank, refill it, cycle it, add fish.

The good news is that the back wall held in place, and is firmly attached. I don't think that it's going anywhere. And, the tank doesn't leak. I was worried about spray foam expansion damaging the glass seams, but, they're OK. And, the epoxy shine disappears underwater.

I'm almost there folks. There will be fish in it soon!