:welcome:@roose.
@rickcorey is exactly right concerning what 800 lumens actually represent. It is important to know what the lumens are that the fern(s) is actually getting. Other than staghorn and elkhorn ferns, I personally don't grow the kind of ferns that you are probably referring to. The fluorescent lighting I use is only to supplement light during the late fall and winter months. My fluorescent fixtures produce so many lumens I have to actually get those fixtures a good ways away from my plants. Each fixture produces approximately 30,000 lumens right at the tubes. Two feet away, and those lumens are about cut in half but that 15,000 lumens covers an area of 4x6'.
All I can tell you is to try to put your fixture as close to your fern(s) as possible. Those low-lumen LED's produce almost no heat. The downside is that the closer the LED are to the plants, the less coverage there will be. It becomes a trade-off unless you have multiple LED fixtures. If the LED is pretty much your sole source of light, I would burn that LED 12-16 hrs. a day.
