Help, I have a dracaena marginata I bought a couple months ago. All of a sudden the leaves are turning yellow and droopy and falling off. I don't think I am watering it too much. I usually wait till the top of soil is dry before I water it. I have it by a window but has no direct sunlight. I read that fluoride in the water can do this. How do I check/ fix the ph? Any suggestions would be much appreciated. Thanx!