Attach the wood to something heavy ie stone or slate. I'm peed off with the number of times my wood has moved because I've snagged it when doing maintenance and It never goes back to quite the same place. I got some nano redmoore root(for my 16l) from thegreenmachine and have tied it to a heavy piece of slate. I wrapped the slate in a hair net first the tied the wood to the hairnet and then ran some nylon over the top of the hair net to tighten it all down. I then covered with the substrate. Now when I knock it it has some movement but goes back to its normal postion. Now I just have to do the same with the wood in my other tank but will have to wait as there is root tabs in the substate and I'm scared of disturbing them!