Are you using an ec meter?
I’d suggest you gently flush the coco with 0.5ec until ur run off ec is within 0.6 ec of ur input solution. (In ppm’s on the 0.5 scale, say ur flushing with 250ppm, ur run off shouldn’t exceed 550ppm. If it does, continue till it comes within range). This will make sure any salts that have been building up are flushed out of the medium.
Because your lower leaves are yellowing and ur plants looks hungry, once uve done the flush, id suggest you give the plants around 1ec of nutrient solution to get them going.
Always water until u get around 20% runoff. Its called coco dtw for a reason.
I hope this helps.
Edit: Also, id suggest you go for gh 3part and their calmag. It’s very simple.
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