For anaerobic nitrate reduction a porous media like:live rock,a sand bed or Seachems matrix and other pourus media is usefull.
K1 is not porous;it supports surface area exposed to high flow like bioballs do, maximizing O2 availability for ammonia oxidizing bacteria that colonize it's surface,providing a good environment for these nitrifying bacteria as they oxidize ammonia to nitrite and then nitrate.
It is not useful n for anaerobic denitrification which occurs in lower flow and lower O2 environments where the bacteria use the O from the NO3(nitrate), when the O in the surr9ounding water is used up, leaving N some of which joins with N forming N2 nitrogen gas which bubbles out of the water.