Giant hygro question

I have a large stand of H. coryombosa var glabra (think I got the spelling
right) in my aquarium. I started with two stems and it has grown and
multiplied very well. In fact it needs trimming about once a week. However,
it seems to grow the leaves quite far apart and looses the lower ones
quickly. It gets lots of light (160 watts over 48 gallons) and plenty of
nutrients. The leaves go very red at the top. So what's the problem? How
come it won't get bushy? Does it need a lot of room?

in Vancouver where it's too hot!