I think you will be waiting a LONG TIME for that driftwood to sink. I had some driftwood soaking for more then 6 months and none of it sank. Some of the smaller pieces stared to float less but none actually sank to the bottom. I would recommend going to a hardware store or a landscaping place and getting some slate tiles or shingles, a masonry drill bit and some stainless steel screws. just break the tiles with a hammer to an appropriate size, drill a hole in them , screw the wood too it and Bob's your uncle. It shouldn't change your water chemistry and it takes WAY less time then waiting for the wood to sink under it's own power.