HELP!！！！

1. My plant is 45 days into veg under a 400w MH .
2. Watering 2 times a week, 25c/50% Humidity

the old leaves turn yellow! and the tip of the leave dry out and droped down。

WHAT MISTAKE I HAVE MAKE?

somebody help!

hahahihi.... 45 days into veg? and its in flower?

Congrats Its a flowering girl, but not the cool hippy chick kind.. more like one you'd see at the end of a slasher movie all cut up and almost dead. That is just sad sir. I'm going to jump out on a limb and say "Bad soil, Subpar lighting, No nutes, and WTF". Obviously its in flower so the light cycle is too short for veg.
Take your loss, read a book, and start over... Try, Try again, practice makes perfect.
Longer light cycle(18-24hr) for veg, Get some decent soil and don't top dress it with dead whatever that is. Get a good feeding cycle down water/nutes/water/nutes not just water. I'm guessing your only watering and not feeding from the looks of them.

NPK

If at first you fail. Try again. You'll get better over time. Just keep it up and you will be growing dank in no time. Best advice I think I've ever been givin is. " don't over think this. It's easy to do. I've over thought much of this stuff and it doesn't help. Have fun with it.

What are you growing in?