Can I cap sand over a gravel cap on a dirted tank?

[PattysAquatics]

Question, I have some dirted tanks and they are doing good but have considered adding more of a cap. I have about 1-1.5 inch cap currently some with sand and some with gravel. I was considering putting sand over the gravel caps . Is this doable to make the dirt level anoxic to get anaerobic bacteria? I obviously don’t want to remove gravel over dirt to put sand. 
These tanks have been up and running for a year and a half and are doing great just want them to last long term and feel this might be necessary for that to happen. 

[nabokovfan87]

I watched a video today and their recommendation (Filipe Oliveira) was to push the soil to the side and add the sand so the sand is directly on the base of the tank.   I use flourite black as a cap on my soil and it generally works well for me.