You should slow down on your feeding judging from the darkness of that leaf.
Use what's recommended on the bottle. It's usually only 0.5ml per liter. Get it on ebay if you can't find it in the hydro store. Go with Calimagic if you do use ebay,
it's the cheapest and there's less nitrogen in the bottle than other brands which is good. In my country they sell a product called flairform cmx which is cal/mag + all
trace elements so it pays to do some browsing, not all cal mag supplements are called cal mag.