Nutrient burn or my water? (1st time using coco)

My question to some one is, what type of nutrient dificiency turns the older lower leaves yellow, curls the ends of the tips and burns the tips as well? This is my first grow using coco coir. Ive never had any issues with soil,rockwool or promix before so I was wondering if I needed to add some Cal Mg to my water and will that fix my problem?

Sorry no pic yet I will throw one up as soon as possible because it's hard to tell how to fix something without seeing the actual problem.

Whatever the main problem is, it sounds like a macro deficiency and so the CalMag won't fix it. Once you post pics, people should be able to tell you which macro(s) is causing it.