Deep sand beds are a topic of much debate. Some people love them for their denitrifying capablities, but some people have had problems particularly if the sand bed becomes stagnant once it is a few years old. That is why some go with bare bottom tanks or a shallow sand bed. Personally I have a DSB in my pony tank and my fuge, but a SSB in my display. i figure this give me a good combination of the two methods.