I'm new to this forum and I'm sure this question has been answered many times. I'm going to be adding a large piece of driftwood to my 180 gallon tank. After sinking it in the bathtub for several days, it still floats. I've heard that it is possible to silicone driftwood to the bottom of the tank, but I think my wood is too big for that. I'm thinking of building a cardboard or wood mold for the driftwood to set in and then pour concete of plaster of paris around it. Once it sets up, the driftwood would be reomved from the mold and set in the aquarium. Is concrete or plaster of paris safe to be in the water - could it change the water's chemistry?