Leaf tips turning brown and dying

[Uncle Zhorik]

this is royal dwarf automatic
its 26 days since i germinated it, under 20/4 cfl lights
i water it every 3-4 days
temp at night - not less than 19C
temp at day - not higher than 31C
edit: water ph: 6.7-7
i have fertilized it only once, with nitrogen rich fertilizer and havent flushed it
the leaf tips are becoming brown and dying, its spreading to the center of the leafs
can the fertilizer be the problem?

 

[JJ Bones]

Do you know your pH?

 

[Uncle Zhorik]

6.7-7

 

[JJ Bones]

Are you feeding any calmag? Is that the pH of the water you're using?

 

[Uncle Zhorik]

i think that none of my fertilizers contain calmag
and its tapwater (original ph: higher than 7.6) with added ph down solution (phosphoric acid)

 

[cannasensei]

Look like ur in coco? If so ur Ph is way off

 

[Uncle Zhorik]

its premixed peat,tuff, coconut fibers, aeration material and controlled slow release fertilizers(thats what i read on the sack of my soil) + added some more coco fibers by me