Decoration wont sink, is adding a weight safe?

[MonteCarlo]

My plastic cave wont stay down since I didn't secure it well enough into the substrate. Id rather not make a mess out of the substrate smushing it in, so would it be okay if I drilled a screw into the inside of it and hung a rock with say... a metal wire? Or would string work better?

Im saying this b/c idk if adding the metal screw or wire will mess with the PH or something. If this is an obvious question I apologize haha. And yes it will be hidden so it wont look bad.

[clopez1]

Turn it upside down to let all the air out that is under it..It will sink.