I am one of the people who will advise against using sand with AFT's.
I do not agree with the use of sand, even with a humid hide for african fat-tails. These guys are not found in arid conditions, sand is not natural for them. So why use it?
AFT's are found in West Africa (Nigeria to Senegal). The climate for these countries is a tropical one. Granted there is a wet season (hot and wet) and a dry season (hot and dry), but it is still tropical. Only one part of Northern Nigeria is arid but this is not in the real range of AFT's.
If you really want to use sand with your AFT's then mix the sand with bed-a-beast or something similiar to make it a "sandy soil, type" mix. This helps hold the humidity.