I agree with Kathy that if you keep it upright, the rim is distracting. The two sharp pieces are interesting and give a real edge to the aquascape that could really make an interesting tank with the right plantings.
However, I'm looking at that large piece and wondering what it might look like if you lay it on it's side, like it is in #3, and rotate it slightly towards the back with the soil sloping greatly towards the back enough that the back short piece (that ends near the middle of the piece) actually ends going into the soil. I think the stump itself would work like a ledge to keep the higher soil in the back, with a shorter level of soil in front of it.
Then, loose the two, shorter sharp pieces and bring the other arching piece so it looks like it's coming out of the shortest part coming out of the stump.