MR1 Reactors are multi-functional.  Some suggested uses include:

Phosphate Control Media

Nitrate Control Media
Activated Carbon
Fluidized Sand / Substrate for Bio-Filtration
Sulphur De-Nitrification (Experimental)
Desiccant Air-Drying for Ozone Systems
Calcium Reactor Effluent Treatment / 2nd Stage

Every MR1 package includes a reactor, 90 degree hose barbs, heavy-duty filter sponges and plastic hose clamps.  The MR1 Reactor also includes the Hang On Adapter.  (Note: Blue / White sponges picutured have been discontinued).

Recommended Tank Size: 250 gallons Marine / 500 gallons Freshwater
Height: 16" Overall Height / 14" Reaction Chamber
Width: 6" Overall
Reaction Chamber: 14" x 4" 
Total Capacity: 155 cubic inches / .68 gallons / 86 fluid ounces
Inlet / Outlet: 1/2" 90 Degree Barb Fittings - fits standard 1/2" ID tubing