I only dose ferts when the test kits say I need to (or when I have a proven, established pattern of dosing that keeps the parameters in range).
As for the LED, shoot for something in the 10-watt range, as long as it has about the same number of LED as it has for a watt rating. Meaning, it's using something similar to 10 "1 watt" LED emitters. Even 5 watts of LED light may be plenty for java fern/moss depending on how efficient the LED is, but 10 watt is a bit safer. You can always raise the LED up a few inches to lower the light intensity.
Something using 3 of the 3 watters is fine also.
The reason I say this is because many LED lights will have DOZENS of tiny LED emitters that consume only about 1/3 watt each but they are pathetic in efficiency. The 1 watt and 3 watt emitters are almost always much more useful and efficient, and predictable.
You should be able to find a good light to fit an incandescent edison-base (common light bulb) socket at Lowes. Look for 6500K, consuming NO MORE than 10 watts, with the widest possible light angle (60 degrees or larger, or else you'll have a big spot light).