Heteranthera zosterifolia and Stargrass is the same plant. The lower plant is mid green to dark green but the growing point is yellow or light green. This plant is fast growing and any lack of iron will show up first in this plant. If you see most of the top is light yellow it is an indication that it lack iron. Use a table or stick iron fer to insert near its base. Insert deep cause this plant have deep long roots. Young leaf of the plant is a bit pointy, as they get older they become rounder. Planted in groups with a ladder style from back to front will created a very nice bush effect. On low-mid lighting, the spacing between the stems is wide while in higher light the spacing is shorter. It grows fast (you've been warn) and branch very easily when you trim them.

Frankie