after 5 pots of HC and 3 months of trials and errors, i finally got mine to grow. in my experience, the most important factors in getting it to grow, in this order:
1) firm, nutrient rich substrate. your best bet would be something heavy that comes with plenty of nutrients, like Aquasoil or Eco-Complete. mine didn't start forming many runners until the Fluorite was packed with fish waste (3 months).
2) it needs a few weeks to adapt to your tank. many nurseries grow their pots emmersed so they tend to melt for the first couple of weeks in your tank, esp under high light. i would recommend reducing your lighting to 2 WPG and dose small amounts of fertilizer during this period. once they have stopped melting and slowly form runners, increase your lighting, CO2 and ferts.
3) even though HC will grow without CO2 and just 1.5 WPG (in my shrimp tank), if you want a full, nice carpet in 5 months you'll need something like >3.5WPG. CO2 is really not that big of a deal. of course it helps, but high light and adequate circulation is a lot more important when it comes to having a nice and healthy carpet. 1-2 BPS is plenty.
4) if you get any kind of algae, turn off all the lights until they are all gone. as long as you have algae, HC will never grow the way you want.
my tank is 75G. it has Flourite, 3.8 WPG and no CO2, no ferts. but HC is growing very well, covering every corner of the tank because of high light and a thick layer of fish waste at the bottom.
