Sounds underlit and underfed. I have a similar setup on my desk and
use a 13 watt daylight fluorescent and the moss grows ok.[1] With an
18W (screw in) it gorws better but it's a bit glaring so I backed it
down.

Get some Tropica Master Grow. That'll feed the plants.


[1] You should see this stuff bust out in a well lit highly
fetilized tank. Yow.

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