Was asked for more info on this so here is a link to the first test video.
https://www.facebook.com/dan.rempel.54/videos/1248323501881586/
And here is the process I used to make it but I didn't start taking pics till I was half done.
I will add the pics at the end.
Materials: All materials I bought at Rona except the ones I have listed elsewhere
1. 1 Tube GE 1200 silicone (I get this stuff from Ewing at Fish Gallery)
2. Carpenters Glue (Wood Glue)
3. Styrofoam (I used both the Blue very dense stuff for insulating 1 piece 2'x4' and White stuff that breaks up and look like it is made of tiny pellets bought a pack of 48"Lx14.5"Wx1.5"thick)
4. 1 Bag of QuikWall by Quikcrete
5. Quikcrete Acrylic fortifier
6. Quikcrete Charcoal cement coloring
7. Acrylic white paint (small container from Micheals craft store)
8. Air pump, tubing and diffuser (Pet Land)
9. National Geographic brand white substrate (This is important as I tried fine white sand and pool filter sand but this worked the best for me aesthetically and functionally)
10. Drylok Clear (I had to go all the way to Selkirk Home Hardware to get this stuff and its not cheap about $65/gal)
11. 1 PVC 90 elbow
12. 1 length of PVC pipe (Important!!! get PVC not ABS because the ABS can leech chemicals into your water)
13. 1 Can of Good Stuff spray foam
Tools:
1. Soldering Iron
2. Regular fork out of the kitchen
3. Butane pencil torch (I had my kickin around but they sell them at Princess Auto)
4. Olfa utility knife
5. Paint brushes (small and large)
6. Sanding block (Fine grit)
7. PVC cutters
First I cut triangle wedges out of the white Styrofoam 14.5"x10" as I was making mine in the corner of the aquarium. My Aquarium is 24" high so I cut 16 pieces. I glued them together to make a 24" high wedge 14.5" along the back and 10" along the side that I could begin carving out to make the desired shape for my waterfall. Using a kitchen fork I dug into the Styrofoam and then pulled towards myself to break off the chunks slowly working toward the design I had in my head. If you take off to much you can always glue a piece back on and let it dry then keep going. After I had basic shape I wanted with the channel dug out I used the Butane torch to smooth over the surface and melt in some recesses in the Styrofoam to help give it a more natural look. I also included 3 dug out pits in mine so that I can put some Aquasoil in after for some live plants as you will see in the photos. I then cut out a channel about 1.5"x1.5" from the back corner and bottom of the foam to place the PVC pipe and elbow into the structure as the pick up for my sand is about 6 inches out from where the sand comes out at the top. I cut and fit my PVC pipes to 22" and 6" long and then connected them with the 90 PVC elbow I sprayed about 1/4 of spray foam in channel I cut out and pressed the PVC pipe into the foam then put another layer of spray foam about 1/4 over that (too much is good to little is not). After the spray foam dries about 8hrs you cut the excess off flush with the Styrofoam then glue then cut the blue Styrofoam so it will cover both back side and glue it to the back sides.
I then used a soldering iron to cut the blue Styrofoam off exactly at the edge of the white Styrofoam so it was nice and uniform. Now the time consuming part begins. I did 3 coats of Quikcrete waiting 24 hrs between coats. First you need to sift out the fiberglass re-enforcement they put in so that it doesn't look like a furry rock after with strands sticking out everywhere. Then mix equal parts of water and Quikcrete Acrylic Fortifier and add the sifted concrete until you get a smooth mixture just a bit thicker then the consistency of paint. Then using a paintbrush paint it on covering everything except the back 2 sides and bottom. Let cure and on the second and third coats repeat the same but add the Quikcrete coloring. The using a sponge I used White Acrylic paint to paint on what look like wear on the rocks. Simply dab a sponge in the paint (pick up as little as possible or dab after on something to make sure you only have a little) then lightly drag it over the concrete it will highlight just the portions that are sticking out further creating a worn kind of look to the rock.
Now on the last step sealing the structure. I used the silicone to seal any cracks or gaps in the seams of the styrofoam on the back or bottom. Then applied 3 coats of Drylok clear to the entire structure. Front, back, bottom, top....everywhere. the reason being I don't want any influence on the hardness or chemical makeup of my water and the only way is to make sure it is completely sealed. I used a small craft paintbrush to do the first coat so I made sure I didn't leave even the tiniest pin hole. After 3 coats I siliconed it into my tank making sure to not be cheap on this after all it is Styrofoam and will naturally want to float. I did a zig zag pattern on both sides and the bottom. After I let the silicone ccure for a couple hrs I went and silicone all the edges and top and bottom to completely seal it. After waiting 48hrs I installed the airline tubing and air stone (longer and skinny is better as it will obstruct less of the pipe and allow better flow of water and sand) filled the tank with water and added the sand slowly so as not to plug the vacuum created by the rising bubbles. If you do plug it a turkey baster squirt in the bottom intake makes quick work to unplug it. The final product is that sand fall in the murky tank in the link of my first post (you may want to wash your sand thoroughly before putting it in the tank as you can see what happens in my video if you don't)
If anyone has any questions feel free to ask and hopefully I have the answer
