If your water maintains a pH higher than 7, lead is probably safe, but controversial. If you pH drops to 7 or below, definitely don't use lead. I have used lead-free solder to weigh down driftwood. It is a 95% tin alloy that is nontoxic. You could also buy bismuth shotgun pellets from a gun shop (reloading supplies), as they are even denser than lead but really expensive. bob