Reactors just hold filtration media in place and pump water through it. Sometimes it's granular activated carbon (GAC), sometimes it's granular ferric oxide (GFO), sometimes it's bio-pellets, sometimes it's even a macroalgae like chaetomorpha.
There are other things called "reactors," like calcium reactors or kalkwasser/Nielsen reactors, but these are a bit different than the reactors mentioned above. They're designed to hold media, like calcium carbonate or calcium hydroxide, but they need to do more than just pump water through. In the case of calcium reactors, they need to control and regulate how much CO2 goes into the reactor. In the case of kalkwasser reactors, they need to stir the calcium hydroxide slowly and periodically to ensure full saturation.