Originally Posted by mr_convitbau
The tank is about 15" tall. Light is PC.
With a 12" space above the tank and 15" to the bottom, you might be pushing it with 27" between the PC lights and the substrate. I would try lowering the light a bit if possible. If conditions are less than ideal it's always easier to grow hairgrass if you plant in smaller groupings of maybe 4 or 5 leaves. Ths allows more light to get to the plant. Also if you didn't do so trim the plant IMO this helps the plant spread quicker.
And yes if conditions are good the plant will spread without having to uproot and move.