Its basically caused by a nutrient inbalance in what ever you are feeding your plants, it could also be caused by lack of co2 if you have a highlight tank, or if you do use co2 maybe you arnt getting enough to dissolve in the water.
So it can be a few things i suppose, what is your setup, lighting WPG and what do you dose as fertiliser, also nitrate levels and phosphate if you know them, any info you can provide really to give us a better idea of where the problem lies, there is no single answer really it could be a combination of things.
