If you are worried about the rocks falling off the driftwood, and causing problems, take HarleyK's advice and 'tie' the heavy object to the driftwood.
If you are unsure of a type of twine or rope to use in the water, and unsure if it is save.. (ie: Plecos eating rope is no good).. you can use airline tubing, as that is what I have always used to tie off driftwood.
If you are unsure of a type of twine or rope to use in the water, and unsure if it is save.. (ie: Plecos eating rope is no good).. you can use airline tubing, as that is what I have always used to tie off driftwood.
too soft. Titanium alloy!