Wow, I was looking at your pics at photobucket, that is an amazing set up with the trunks within the background! Where did you get them trunks? I think I'll definitely try me some of that mortar!
But if one doesn't use a sealant, what would you suggest be used as a core frame? Styrofoam? Steel chicken wire? You know, so it doesn't end up weighing a lot.
I used pink insulating foam, the kind that you can buy at Lowes. I had a whole bunch of it, one of my friends had a bunch of scraps from something theater related (building sets or costumes or something).
I got the wood from a lake in Kansas. It was during a drought, so the water level was low, and the pieces are basically previously submerged bases/stumps from medium sized trees. I brought a sawzall and measuring tape and cut off some good pieces. The middle 2 pieces of wood in my background are actually the same stump. I cut it in half down the middle and placed the flat part against the background to make it look like stump/roots coming out from a river bed. I siliconed the wood to the background to make sure it would sink and to make the entire background 1 big piece. I figured out this may not have been a great idea when I had to get a big pleco out of the tank. I couldn't move any of the wood, so it was really hard to get him out of hiding spots so I could net him.
The background weighing a lot is not a negative. I had issues getting the big pieces to sink, although that's mostly because I was working with acrylic. Silicone holds a lot better to glass.
Think of it this way, a basic 2 foot X 2 foot foam background that's an average of 4 inches or so thick will displace almost 100 lbs of water, which means that the silicone holding it to the tank has to hold almost 100 lbs.
I like the concrete/mortor because of the texture. Epoxy has a lot of advantages too, and adding sand or gravel to the mixture can make the texture to be almost whatever you want.
Overall, making that background was REALLY time consuming, but it wasn't that hard. Just come up with a pretty good idea of what basic design you want, and then carving it isn't that hard. The concrete or epoxy will create the texture, the foam only needs to make the basic shape. I'm an engineer, not a very artistic person. I was surprised how easy it was to make the foam look like rock. For the rounded parts I used a dremel, which was messy, but did make nice river-rock looking formations.