Are you talking about amphipods? In that case a deep sandbed wont do any difference. They primarily reside within the rockwork and it's almost impossible to eradicate them, once they are established. On that note however it still takes close to a year, before the population can sustain a single mandarin fish. Also you wont reach that sized population without feeding your other tank inhabitants.
It goes like this: You feed your non-predatory fish -> leftover food gets eaten by scuds, pods, etc -> their populations increase to a sustainable high number -> you can now have a single predatory fish living completely off of the pods, as long as you keep feeding the other fish.