Your return pump is by far the most important piece of equipment on your aquarium. It circulates the water which is basically the life-blood of a tank. Creating a great aquarium is just like building a great body â€“ it all starts at the core â€“ and this is precisely how our pump got its name! The COR is a variable speed, brushless, DC-driven centrifugal water pump designed specifically for aquariums to be intelligent, energy-efficient, unbelievably quiet, and incredibly powerful for its size. Â No Apex neccessary.