The reactor is simply a chamber to put media (whatever you want: carbon, GFO, etc) into. You then attach a small pump to the reactor which pushes water through. It's more effective than just a filter bag because water passes through and all around the media evenly.
Most people use it for carbon or one of its substitutes, or granualated ferric oxide (GFO) which removes phosphate.
A handy little device although you can't put a large amount of media in there for big tanks (clogging issues) and getting the right amount of flow through the media can be a little tricky especially if you've hooked two up in series.
I also have one hooked up to my Tunze osmolator water top-off to dose kalkwasser.
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