The density on the gravel is greater than the density of the Garnet, Silica sand and Anthracite. Also keep in mind that the back flow is intended to "lift" those three layers and the "cake" formed during the filtration while the gravel will stay at the bottom.
I found some information on the same sobject:
NATIONAL SPA AND POOL INSTITUTE
Suggested Chemical Standards for Swimming Pools
Free chlorine, ppm 1.0 - 4.0
Combined chlorine, ppm None
pH 7.2 - 7.8 (ideal range of 7.4 - 7.6)
Total alkalinity, ppm (for liquid chlorine, cal hypo, lithium...