Tipburn of hydroponic lettuce can be caused by a variety of environmental factors. Inner tip burn can be caused by lack of calcium in the young developing leaves which may be due to environmental factors. Outer edge tip burn can be caused by high soluble salts, a result of different environmental factors.
For more information see: Tipburn of Hydroponic Lettuce, http://www.e-gro.org/pdf/2015_431.pdf