Well, I've just put a bag of Salvinia Natans in the tank and it's covering about 50% of the water surface (all down on the left of the tank thanks to my flow) and I've got another bag that should give me full coverage of the surface if I decide to use it.. I can't say that I'm particularly impressed by it though, I was under the impression that the leaves would be joined together by 'runners' (not the right term probably) but they're mostly individual or in small clumps (three or four leaves).