A rule of thirds could serve you well. Pairs and symmetry look odd. Proportion is everything.
There are too many rocks IMO.
I'd leave the stack on the right, but tilt the peaking rock in the other direction if possible.
Then imagine a triangle with that right peak ... Extend each leg of the triangle to the corner of the tank.
Then take out most of what you have put on the left, and make it more irregular. It should sort of complete the one big triangle.
That's the best I can explain without drawing or knowing the dimensions of the tank, lol.
The right stack should be more to the back and the left more to the front. Imagine a similar triangle from above.
And it should look pretty accidental.
Sent from my iPhone with three hands tied behind my back.