I don't know for sure about the lead, but some ideas here...many places selling any form of slate and rock that have many small pieces that I'd bet would let go for free or very inexpensive - removes any doubt from the equation if you have any. Or, depending if you have certain amount of rock Su substrate you could you acrylic platform. You could attach with some pieces of acrylic rod to attach together or superglue. Glue will hold it and not harm fish. This will make it appear the wood is just syncing at the bottom. It's more effort but depends on what your willing to do to get the end result your looking for