new growth yellowing

i have a 10ft by 5 ft room and the temp sits at 75 and humidity at around 60% i have a good in and outlet and using a 400 wat hps. i have a few cuttings freshly potted in plagron batmix and i have 2 3week old plants which stand about 35cm, i noticed today when i fed the 2 older ones that when i fed them, the leaves drooped within 3 mins, i didnt use any furts because i freshly potted them into plagron batmix about a week ago and was told not to add any furts for the first 4-6 weeks, and i recognized slight yellowing in only the new growth coming through, the nitrogen in the soil is week and i was thinkin its prob a nitrogen defencie??? i ph at 6.0-6.2 any help would be gratefull!!!!

i forgot to mention that i was boiling my water 24 hours before feeding about 2 weeks ago but i was told i didnt need to, could it be a water problem?? shall i boil my water in the next feed???