The rock will obviously weight a lot less in water, the weight of the water displaced can be subtracted from the rock weight to get the apparent. The density of rock varies but it's often not much heavier than the water itself. Actually recently seen a piece of rock float in a tank
So basically don't be concerned about the weight of your structure on the glass, it's also much stronger than the plastic PVC frame you built.
The structure you made looks awesome btw.