You grow in coco,leaves showing absolute magnesium deficiency,my bet is you didn't feed each time you water up until here.In coco,try to feed each watering.its not like soil,it needs to be fertilized any time you water it.especially calcium and magnesium are lacking.burns at leaf edges are caused by skipping feeding and giving only water.they surely had some amount of exces nutrients and when you stop adding nutes to your watering,they tried to absorb excess minerals with that nuteless water,amount of water is not enough for them to absorb completely and this happens.experienced it before,a few times and I'm pretty sure about it.feed each time and add some magnesium.if you see serious nute burn on your leaves,flush once only,then add some nuted water again.