2-3mm sand,
But using something like Flourite, Onyx, Ecocomplete, profile/MVP turface etc would be even better.
Once added, you seldom ever want to mess much with sand.
Play sand generaly is too smaller(1mm less).
Add fresh Mulm and Peat to any new plant substrate no matter which method/type you chose.(search for these on the substrate post)
Regards,.
Tom Barr