I have acquired some plants out of the blue and don't have a good background on how they were grown previously. They are in soil, but apparently were not fertilised; I have only watered (added no nutrients). They have been in flower for a few weeks, but are showing minimal progress. I had them under poor light for the first 1.5 weeks, but now have good light.

The plants to me appear rather tall and thin stemmed, as well as overall rather dark green. All plants are showing signs of stress in their leaves, some much worse than others - the worst ones have dry curled lighter colored leaves. The healthier plants still have dark green leaves, but are slowly progressing to having the leaf tips point down (or in some cases up). This affects upper and lower leaves, though the upper leaves more quickly progress to dried discolored (brownish) leaf edges and then to curling .

I've researched and checked the search function, but I don't have enough experience to know for sure what my problem might be. I've grown other kinds of plants before, and am confident my watering is not the problem. I've checked the roots of a few plants by gently pulling the wad out of the pot; they could use bigger pots but the roots are healthy looking and white.

I've attached pics of some of the worst leaves; this is how they end up looking before drying out even more. The pictures lit by natural light from a window. Am I on the right track as to my problem, or could it be something else? I had planned to flush the soil; will this help?