I agree with others. I have now had play sand in one tank for six years, and in the others since then as I changed them over from fine gravel. Any substrate will develop anaerobic pockets, and in fact this is essential to a healthy aquarium. The substrate under chunks of rock and wood for example will be anaerobic. What you don't want with any substrate is for the entire area to become compacted and anaerobic. The other members have set out the ways to avoid this.
I have some tanks where I never dig into the sand, others I skim over the surface stirring up no more than an inch, in open areas. I happened to tear down my 33 gallon longest-running sand tank last summer, as I wanted to set it up as an authentic Sri Lanka stream for my Black Ruby Barbs. I pulled out all the wood and plants (what a mess, naturally), and then cleaned the 1.5 inch deep sand substrate simply by poking into it with the Python. I was quite surprised that I didn't find much in the way of anaerobic pockets after so long with no disturbance.
Byron.