Putting a layer of corse filter mateial (pea gravel) in befoe the fine filter material (sand) is an effective way to filter and is pretty much standard in any aquarim filter setup. The sand will not "fill the holes" but will pretty much stay in it's own later. This will allow a better water flow through the laterals with less chance of them plugging with sand, will help reduce the chance of lateral breakage, and should give you more effecient filtration overall. It probably is not necessary but IMHO is a good idea. I am wondering if anyone has ever done it both ways to check the difference in a pool filter. BTW, sand filters similar (sometimes identical) to pool filters are used to filter large commercial aquariums for mechanical filtration.