A modular aquarium filter includes one or more cartridges connected in series. The cartridges may contain filter media for chemical, biological and/or mechanical filtration. In addition, the cartridges are designed such that they provide a unique internal helical geometry to increase the contact area and flow rate of the water with the filter media. The helix may be optionally adjusted to provide different thickness filter layers for aquarium filtration. The pitch of the flights of the helix may be varied to provide enhanced centrifugal flow of the water within the cartridge. The present invention also provides self-sealing valve assemblies at the filter inlet and outlet, which when disconnected shut off the flow of water through the filter to expedite exchange of the cartridges.