You may be correct in your thought that you don't have enough lighting. Please begin by telling us more about your aquarium (i.e. volume, lighting, pH, KH, GH, NO3, PO4, K, etc.). I would imagine it is a lighting issue, but without more details, it isn't possible to give an accurate assessment.
i use now 3 wpg.no the whole thing is turning brown.i add some iron and potassium from seachem.ill wait 3-4 days to see what happens.i cant say the exact parameters you ask but for starters i have 6.8 ph - 3 KH - about 20 nitrates-temprature 76 - 3 wpg-and the tank volume is 29 gal but a little tall.(19 inch)
When you say you add iron and potassium from seachem, you mean Flourish Iron and Flourish Potassium right? What about Flourish itself? If not I would suggest starting with that. Otherwise we may need to know more details about your tank. The fact that it's the tips showing such problem I would exclude lighting as being a possible cause.
Is the riccia floating at the water surface, or is it tied to something below the water surface? I ask because the tips may be getting scorched by the light if it is floating on the surface. I had this happen once when I let some riccia touch the glass cover on my aquarium, which was apparently hot enough to burn the tips of the plant. Just a thought.