Yellowish tip of Cucumber Seedlings | Vegetable gardening | Commercial Hydroponics farming in India

Hi,
I am new to gardening and have grown cucumber after seeing the sowing chart for North India. The plants are about one month old and are stagnant since last two weeks and now have developed yellow leaves at the tips and even the color of some leaves is very dark green as compared to normal. As the new gardeners do, I was too much caring initially, so it could over watering, over fertilizer, etc. or too much heat , I am not able to understand.Please suggest the problem.