What are you growing in? Is that actual soil or sphagnum peat moss? If you are sure it's not light/heat burn, then the tap water did it. Last month, my beautiful pepper plant that was growing great for about 6 weeks, i gave it just tap water as its water one night as it was already there and i didnt feel like mixing it with nutes, the next morning, the top of the plant was all fucked up! I threw it out a few days later as it stopped growing instantly and scrunched up at the top. Tap water is naturally ph'd high..above 7 usually closer to 8 to preserve our water pipes. Without nutes or a ph down, the tap water on its own, has a ph that's too high and will lockout many nutes..especially if you're growing in something soilless like peat moss or coco.
i had a small issue with molasses the first time i used it as well. It's quite high in potassium and iron. If using molasses, make sure you dont add too much potassium or iron from your regular nutes with it. Though this issue, unless you put maybe a tablespoon of molasses to like a litre of water, shouldn't have been the culprit.