Hi and welcome to the forum
Plastic sheeting won't help level the floor for an aquarium.
Carpets aren't normally a problem for aquariums unless you spill water on them, then they go mouldy. But the weight of the tank and stand should squash the carpet and level it out.
Assuming the floor is level, there should be no real need to add or do anything to the carpet. If the floor is uneven (not level), then you can use a couple of pieces of 4x2 inch pine and put them under the base of the stand. Alternatively, you might need some shims (thin pieces of wood or metal) and slide them under the base of the stand in areas where it isn't flat.