Anachris will grow well without any substrate because it does not set down a real root system; it draws its nutrients directly from the water column. This is why you can float it.
I would not use sand if I were planning on planting a tank. If you just want to keep things like floating plants and anachris, you will be fine to keep your sand. Otherwise, put in some Schultz's Aquatic Plant Soil (aka Profile) or Flourite.