I have been using some in my tanks for about 10 months now. I just jab the ends down into the substrate.
Toni, my Oscar don't mess with them unless I was brain dead and put it in her normal rest area.
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This is when I first got them. Lasted a couple months then it got to cramped for her size and tank size to get around them. So I moved them in with my JDs.
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Had no issues with them trying to move them or anything, again just jaming the ends into the substrate.
When Toni moved into her 125, I brought the branches back in for a while.
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It lasted for a few months till again she started having issues getting around them due to her big size.
I have heard of people tieing them together and to center supports.
What I was going to do but never got around to it. Goto an arts&craft store, get some small sheets/pieces like 8inx8in squares of acrylic and silicon the branches to those. Then bury the acrylic sheet under the substrate. All the substrate weight on the acrylic should keep the siliconed branch in place. Unless of course the fish tips the branch from the silicon, but the chance of that happening.......