Deep Sand Bed. Provides de-nitrafication water filtration. Bare minimum 3", recommended minimum 4", recommended limit 6". Requires fine-grain substrate, sugar-fine (oolite) recommended, some mixed-sized substrate can be acceptable. Requires micro-fauna and other small critters to keep the sandbed healthy. Is a topic of great continual debate. Waterkeeper can tell you everything about it, and would likely be happy to do so.