Those rocks are not going to do anything weird to your water? It probably very minute if it does since they look like small rocks and you have a 36g. How long are you going to soak it for?
I was hoping it would have sank already, but.... its still floating very well lol. This afternoon I am going to drill several small holes in the cut area at the back and then re-soak it for a few days, and if it does not sink then, I will put some stainless steel screws in the bottom and wait. The rocks appear to be safe, and the largest one is around the size of a half dollar, the others are all much smaller. So right now, the only thing I am waiting on is the wood, then it is full steam ahead.