Hi there. I use a mix of topsoil and coco coir and this does hold moisture a bit better. I found the sand separated out and I stopped using it and later introduced some coco coir with the topsoil. Quite a few others do so as well.
How often you change it depends so much on how big your tortoise and enclosure are and how much substrate you use.
Indoors I think there is no reason not to add quite a bit of water to the substrate. Unless your house is a really cold one or the water is going to cause a problem to your enclosure.
So what I do is to use topsoil and coco coir mixed and add quite a bit of water so that it doesn't get so dusty.
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Testudo Hermanni Hermanni (Corse) tortoises
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