Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Sand that is pretty much all one particle size has better water flow than sand of mixed particle sizes.
Sand that is too fine has poor water flow.
Pool filter sand (around here, anyway) is a graded sand, the particles are 30 mesh, or about 1 mm in diameter. There is a small amount of dust, but very little. It works well in a tank as far as good water circulation through the substrate.
Play sand (around here, anyway) is a finer material, almost dust, and has mixed sizes of particles. It packs down in a tank and does not allow good water circulation through the root zone of the plants.
Poor water circulation means low oxygen in the root area, so anaerobic organisms can grow. Mostly we do not want these in the tank. Also, plant roots need oxygen, some plants actually move oxygen into the root area and release it into the substrate.
Sand does not hold fertilizers in a way that is good for plants. Finer materials like clay do hold nutrients for plants.
Best way to use sand:
Not too deep.
Pool filter sand: 1-2".
Play sand: I would not use it at all, but if you must, 1/2", and it is only decorative, not deep enough for planting. Rinse VERY thoroughly to try to get rid of the dust. (And you are throwing away as much as half the bag by rinsing- by the time you have rinsed it that well, you might as well go buy pool filter sand)
Add slow release fertilizer in the substrate. Add fertilizers to the water.