I made my own nitrate reactor (sulphur based) out of an old cannister filter (eheim2217) I had laying around.
You can just buy the sulphur media, and do this with any cannister filter FWIW, its easy, and its not a permanent mod (no drilling or glueing). If you simply loop the output of the cannister filter to feed back into the intake, so the cannister recirculates, it will work as a reactor. I used 3/4" CPVC to do this. Be sure to have two 'T's in the recirculating loop.
Then fill the cannister up with sulphur media from marine-depot, or aquatic-eco (2 places I know have it).
The two 'T's serve as the inlet and outlet for the tank water to get to and from the reactor. You can use any number of JG or compression fittings for 1/4" flexible line... whatever you want.
Turn it on, feed the reactor with a pump or something so it gets a good drip-rate, and you are in business! Its a good idea to use an ORP meter on the output for a bit to figure out what the best drip-rate is.