sand/rock displaces water. even though it weighs more than water, you still have to account for the fact that there is less water in the tank when there is sand or rock taking up space.
I wouldn't worry about it.. these things are designed for this.
54g all-glass bowfront corner planted FW
Red tiger lilly, Rotala Indica, Sagittaria Subulata, Micro Sword, Cardinal Plant
Fluorite Black sand and gravel