I believe the stir plate oxygenates the starter media by keeping the yeast cells in suspension and moving along the surface of the vessel, where the oxygen is. An aquarium filter will act differently, by moving the ozygen across a (smaller) sample of wort. If I recall correctly, one of the brewing pod casts did a test where they did 5 minutes shaking and continuous injection of oxygen from a cylinder for post pitch aeration (for an full batch, not a starter). Shaking did better than continuous oxygen. If you choose to use the aquarium filter,I'd recommend you rig up a filter on the air intake to keep dirt and oil from the motor out of the starter media.