Growth seems stunted?

Some actual pictures of the problem would have helped. The discolouration on the tips of the lowest leaves is usually caused by those leaves either hitting the soil or the edge of the pot when the fan is blowing on them and generally at night when the leaves are lowered. These blemishes will not affect growth, potency or over all plant health and are of no concern. No plant will ever have 100% perfect leaves, 100% of the time. You only have a 90 watt light, so I would not be expecting explosive growth. Whatever you do, do not defoliate any healthy green leaves, this will just reduce your plants capacity to produce energy, thereby slowing growth and leading to smaller plants. The fact this plant may be growing slower than other plants that you have grown could be down to many factors, temps being one, but the main factor is the unique individual genetics of this very seed that you have sprouted and are now growing. Each and every seed is an unique individual being, not a mass produced exact copy of this particular strain which will grow identically to each and every other seed of this strain. It is a bit like people, although we may have the same mother and father, we are all very different to our brothers and sisters! Just keep up your current care routine and in time she will reward you with tasty flowers. Also, cannabis is a remarkably tough and resilient plant and does not need "Princess Perfect" exact conditions to grow properly or successfully. A few degrees here or there or a pH a point or three up or down will not effect your plants growth or health in the least. Plants are not mechanical machines, but organic living beings that have been growing in nature for Millenia and without any help from people, entirely successfully. They do not need little humidity domes or being wrapped in cotton wool to be able to thrive and prosper. It got the name "weed" for a reason. Hope this helps,............ Organoman.