an extended time period of little food/detritus for them to feed on will naturally lower the amount alive. if you add something to eat them your increasing your bio-load and decreasing your natural cleanup crew, and in effect pushing your tank out of balance. In my opinion this is ill advised. I would let the population decrease naturally. but if your hell bent for leather on doing so any sand Sifter will eat them out of the sand bed which again in my opinion is a bad idea as sand sifters will decimate the microfauna populations in your sand bed possibly leading to more annoying problems.