Planted Tank Guru
Join Date: Nov 2009
Location: Missouri united states
If it is just the one piece that is in question,I might place a large smooth river stone on it to hold it down until such time as the wood becomes water logged and stay's down on it own.
Some wood takes longer than other's to become water logged.
Could super glue some anubia,or bucephalandra to the wood and tuck some more plants around the base.
Just an idea.
I don't care for not being able to move wood pieces such as one might not wish to do if drilled or glued to tile under the substrate,but for center piece,or focal point,(not gonna move it),then attaching it to tile as mentioned would maybe help keep it in place.
Larger pieces of wood might take fairly thick piece of tile to keep the wood from wanting to bob back up until it's water logged.
I have used stainless steel screws into two inch thick Sandstone pieces of tile when larger pieces of wood that I tried still wanted to bob back up after eight week's of soaking outdoors in tub of water.
As mentioned,,some wood's just didn't want to stay down for me despite what I felt was long enough time soaking.
I had the idea of the hardscape in mind,,and perhaps just needed more patience.