At least group the 3 littlest closer to each other, and then closer to the mid-sized rock. So there is a triangle: Big rock farthest back. mid-sized closer to center, small (group) about halfway between the largest and mid-size, but closer to the front of the tank.
The way you have it makes a hill way over to one side, which works, but you could also move the rocks around according to the golden mean. Google that, but do not take it too seriously- basically it means putting the major focal point near the center, but not right in the middle. About 2/5 of the way.