When looking at the powerheads that are either in your aquarium or about to be installed, you might notice a portion of the grill that is filled in. This area is purposefully built this way, and serves as a vortex break when the pumps are used near the water’s surface. They keep water from being sucked into that particular area, thereby preventing a vortex that could draw in air, causing the pump to cavitate. When a pump cavitates, it not only blows bubbles all over the aquarium, but is subjected to potentially damaging stresses because the pump will spin faster in the less dense medium. That said, the factory blocked intake could also be positioned to keep delicate animals out of the pump, or to prevent other materials like sand from chewing up the impeller.