Unless you add some source of organic matter, sand 24" deep will not become anaerobic.
If you add soem organic matter, then either you need to have high enough flows into the sand via pore/grain sizing or good root growth that will pump O2 down there.
It's when you load too much organic matter for the bacteria to decompose, that the bacteria remove all the O2 and then it becomes anaerobic.
No organic matter: no bacteria food, no drain on the O2 levels.
Mulm etc and fish waste will load as will folks that add soil etc and not enough plants from the start etc, or folks that add 1/2 dead/dying plant roots into the substrate and then get the nasty, or folks that over feed and the food breaks down in the gravel etc.
It's no mystery why H2S forms.