Carbon has a "service" flow of approximately 5 gpm per ft2 of bed area. This is for the reduction of organic chemicals, taste, odor, chlorine etc. Some chemicals are more easily removed than others like chlorine. Chlorine reduction is achieved at higher flow rates, but contact time is still important. The vast majority of water flow to a house is in the 2-3 GPM range. Rarely does a house exceed 5 gpm or higher. I have made it a habit of checking the peak flow rates of almost every system we touch and I rarely see peak flow exceeding 9-10 gpm. That being said, code says differently. The potential flow must be accommodated, not the actual flow. A 2 CF Carbon tank with a large port valve will likely peak at 15+ gpm without any problem.
Gravel is cheap insurance. Many online companies don't use it in their desire to be the lowest price company especially when people are only looking at price. Most people don't know that the a high quality valve can be put on a piece of junk, low cost tank, cheap risers, low quality screens, lousy brine tank with junk safety floats, low grade medias, all in their desperate desire to be the lowest price. A well built system will cost a little more. Not using gravel in a traditional system design is a bad idea, but it is common since the companies doing it can save 15-30 pounds of shipping cost. When they offer "Free" shipping...
