Nutrientburn?

chlorosis with rusty tip buns ... looks more like a K-deficiency. 5.9pH shouldn't be an issue, but make sure it hasn't leaked lower. 6.3 is fine, though indicates potential of being higher if this is the runoff.. 400tds.. if on 500scale that is only .8ec. This is a very light feed. so, toxicity becomes less likely. i'd up it to 600-750 ppm ("500 scale" TDS equipment)... 120-140 N / 50 P / 200 K / 100+ CA / 80+ Mg / 100+ S -- the last three will vary a bit due to tap water too. you can see my basic ppm breakdown in any week of most of my diaries... i use a 720ppm at full strength. or 1.44 EC religiously get 10% runoff and toss it down drain or outside.. you should never have to flush a soilless grow if you are consistent at this. you will never have any buildup.. you won't have any pH issue unrelated to some odd microbial infestation etc etc... you may need slight adjustments at various stages of life cycle, but you should fertilize with every single irrigation.. this is the primary beneift of soilless grows... ondemand feeding and readily available nutes for fast growth and limited effort from plant. if not from north america, and possibly in NA too, --- https://www.perfectgrower.com/knowledge/knowledge-base/ec-to-ppm-conversion-chart/