Your quite right mate, certain ones dont need capping.... it has to be moler clay for this.... fullers earth isn't as far as im aware and will break down quite fast in water.
The tesco low dust brand is really what you should have got (stuff i linked to) or even sophisticat pink (bigger granules), this is actually moler clay based and as you can see from my tank, it doesn't need capping. It is essentially akadama but without the price tag.