If you use good quality German expanded clay, roughly 50% will float and 50% will sink... Initially.
Put a brick on top of the gravel guard on your siphon kit/drain before turning the pump on. If siphoned F&D, also remove the bell beforehand. Turn the pump on and allow the GB to fill to it's max' flooded water level (ie: about 50mm/2" below the surface of the clay beads, then get your hands in there and work the beads in. The sinkers will sink and the floaters will work their way to the top. Leave it flooded, but work the beads every few hours, as more will have started turning into sinkers.
After 24hrs you will usually end up with around 3/4 of the beads sinking and about 25% that remain as floaters, those beads will never sink, but if you have the max' flood level 50mm below the surface of the exp' clay, the media and your gravel guard will not rise and fall with the F&D cycles.
I've run trials on drums full of water and different brands of clay that I've left for weeks. All the German brands end up with the same result as above, as I'm sure they all still come out of the same factory, just with different brand names on the bags.
Cheap Chinese and Thai exp' clays are another story... a sad one. The best result I've seen was about 50/50 floaters and sinkers after weeks in water. That was ONE brand, all the rest were far worse, with most ending up with at least about 80% floaters and just 20% sinkers. That % will definitely see the media and gravel guard rise and fall with the F&D cycles. A couple of brands ended up with in excess of 95% floaters, that never sunk, even after weeks in water.
Plus, the cheap clays break down, some very quickly. The German clay doesn't.
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