Goin off what UK420show said, it's always handy knowing the hardness of your water. Hard water has loads of calcium and soft has none or next to none. In the uk where I am it's hard, so I never add calcium all grow. If it's hungry, I think it generally starts draining lower leaves, light issues start above. Where did you take this leaf from and where did you first notice it? Google your water company water quality, if it's available, which it should be, where ever you are.