oh, sorry, thanks for correcting me Imbarrie, the sulfur gas is the danger, that needs to be avoided in anoxic situations of zero O2 levels. I was trying to say too much with a single sentence. Let me clarify with what I was trying to say about the intermediate depths of sand less then 6".
There's not enough depth for a gradient of O2 gas to reach hypoxic levels for proper denitrification or for breaking down other larger waste compounds. Therefore, it's not really necessary to try to have a 4" or 5" or even a 3" sand bed, since it wont function as a deep sand bed, but may have partially broken down compounds or even worst, concentrated levels of compounds as they are drawn in. I personally use a plenum under a 6" sand bed to help form this gradient of lower O2 levels, but it never quite becomes an anoxic situation, so there's no danger of sulfur gasses being produced, yet still allows for denitrification.
Therefore, if it is just cosmetic, I'd go with the thinest layer of sand that's pleasing, and avoid hypoxic areas completely.
And yes, I've had 2-3" sand beds in the past, which without sand shifters, became compacted with fish waste over time, and caused a die off when I moved the rocks around landscaping, exposing the layer of anoxic black sand, which I then carelessly scooped up and moved to make a new hill. It smelled swampy but I didn't care. Until my fish started to die half an hour later.