If your even going half of what is says on the bottle your probably overfeeding. I find whats on the back of the bottle is WAY overboard. If I mix just my veg nutes as per the instructions state I'm around 1000ppm. Even half of that would be too much for younger plants. These don't look to young to young ot me and you should be feeding every watering with coco otherwise your practically flushing the nutes every time you just use water and they sit there with very little nutrition until you feed them again.
In coco you should be watering almost every day with nutes until 20-30% Runoff. The medium should not completely dry out, but almost. I think your plant is stressed from too much nutes, then no nutes, then too much, etc. I keep my ppm on the lower side and feed every watering. That's the beauty of coco. You feed often and it's almost impossible to overwater.
As stated above you need an E.C meter or a TDS meter to know how much your feeding them. Without that it's hard to get your feed accurate everytime. It's easy to go over just measuring with a spoon or glass. That's why you always have to check it along with your ph.