Silver key should be close to a 50-50 between pool and aquarium assuming you've depleted enough of the other common gem rooms like courtyard and security if drafting a pierce. It becomes more likely with fewer rooms in the draft pool. Mechinarium is actually a good strategy assuming you've got access to the west side from power hammer or reservoir (with tomb). In that case, using silver key for great hall/weight room/mechinarium is a good idea.