For many people, a diatom filter = a water polishing filter. A big selling point of the Mag is its ability to polish water quickly. Diatom sizes range from 10 to 150 microns. Thus, the Mag's 8 micron cartridge filters out diatoms. I know some people use a Mag with diatom earth to filter out even smaller particles/parasites. Sure, it doesn't have as much power as the Vortex, but the OP's tank is only 15 g.