I've got a very dense e. acicularis carpet, but I dose EI and dump a ton of CO2 into the tank. The result? It's invasive and grows like a weed. I've noticed too that growth tends to stagnate when I limit light; in most instances I run 108w T5 for 9hours/day, with a 3 hour, 108w T5 noon burst. When I forego the noon burst (like on days when it's hot and I'm trying to control temps) for days on end, the growth is insignificant.
Something interesting to note, however. When i originally planted, I planted tiny, tiny plugs - as small as I could possibly manage; I used a 3"x3" section to cover the entire fore ground of a 90g tank 5" deep (ie towards the back of the tank). A few weeks ago I removed a chunk in the foreground which appeared to have some clado stuck in it, and replanted 2 larger sized clumps in its place. I have noticed very limited growth and runner production from these two clumps, and can, with a fair amount of certainty, suspect that if (or when rather) I separate these clumps into smaller plugs the growth rate will increase exponentially.