I don't know about cutting it, some people trim their hair grass carpet down and that probably helps it's density. The trick that works for me, with Lillaeopsis and Eleocharis, is getting enough to plant a checker board pattern throughout a large open area, ~1" clumps an inch or two apart from one another. Add light and nutrients and a thick carpet fills in after some time. If you're letting it spread out from a single clump, you're probably in for a long wait.
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