I haven't used silicone on any of these. They are held in place by the wieght of the gravel behind them & the first one also has a strategicly placed rock in front of the log. By all means though use silicone to achieve the effect you are after.
The log in the pic on the left is actually a root that I bought at my
LFS. All the other pieces of wood are what I have collected from the bush. If you collect your own wood just make sure it has been dead for a long time, has no bark, no sap & is rot free. If you can, collect your wood from the top of hills as any pollutants are generally washed down away from the wood you are collecting. Look for long dead, standing trees, there will usually be good wood on the ground beneath them. I actually brought back about 1tonne of wood because it is ridiculously expensive to buy wood from aquariums in aus. A piece the size of a 2lt bottle would set me back $40-60aus. Seeing as I have lots of tanks I wasn't about to pay for it all.
You can use rock just as well & if you like the look of layered slate, go to manufacturers of billiard tables. They usually have plenty of offcuts that they will be only too happy to give you.
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Thanks so much!!!
I do have some aquarium scilicone... perhaps I could make some, where did you get your hollow logs? thanks!