Bear in mind that im not an expert at all.
I really like the look of the tank, it already has murky look that gives it alot of feeling. I also really like the piece of wood, it shows great potentiel. The half buried slate pieces looks really good too, I hope you can keep them halfburied like that.
If I was you I would focus on the slate and the big piece of wood. It confuses me a bit that you have so many varieties of rock and the petrified wood. The three stones in front of the wood stands out a bit, so does the rock on top of the slate on the left. I think it would looke great with just the wood and the slate.
That is just my opinion, if you feel differently, go with that. It should make YOU happy, not the rest of the world
