There are a few tricks with growing Heteranthera zoesterafolia (stargrass). It applies to some other plants such as Rotala rotundifolia as well:
Stargrass needs a lot of light to stay nice and green and the tips usually only get enough so the rest below that will often turn black or grow rank. Trim the tip and replant as suggested.
Or, Trim the tip and replant then trim the remainder and compost but leave the stem and roots as it will reshoot.
Or, bend the whole pland over and cover it with the media (apart from the tip) and it will throw a heap of vertical shoots.
Or, Pull the whole plant and trim and replant the tip. Set up a plant pot with potting mix and place all the old roots and discarded stalks on top then cover lightly with potting mix and keep warm and moist. You will soon have that many plants to trim off and plant and sell on trademe you wont know what to do with them.