Just had a look and any tank with HC in it has the equivalent of at least 3.5WPG, one small tank had 8WPG lol, why? Ian has grown HC with about 1.4WPG I think? I know the WPG rule shouldn't apply and all that but its just a rough measurement..
Probably to do with growth rates, but there is a threshold point at which plants can no longer photosynthesise, as Ian says HC is pretty adaptable but it does appreciate nutrients a lot more than other plants do, I suspect that it has a higher metabolism than a lot of other species, especially seeing as it grows quite fast.