Tips are burnt, typically by an excess of nutes... no idea what you are feeding or medium you are using, so best guess is to back off a bit or flush and start up again with half strength, and pay attention to new growth. Also looks like you have white spots on fan leaves on some pics, which looks like mites have moved in. If I had to guess, you started seeing yellowing (from the mites), thought it was a lack of neuts, so upped the feed, which led to the burn?
