Oldest leaves turning necrotic at tips & margins

It started when the first true leaves of my White Widow got to a certain size & then turned a light brownish at the tips & then it developed along the margins. New growth is fine to start with but the leaves then get to a certain size & start to turn brown at the tip first & along the sides again. The dead parts are very brittle. It can't be heat burn beacause the oldest leaves near the bottom continue to turn colour while the topmost newest leaves do not.

I have a big fan blowing several inches above the plant, the temperature is about 85F & the plant is about 24 inches away from the 400w Metal Halide. The ph has been about 6.7. The soil is Light promix which is lightly fertilized. The grow room space is 3ft x 3ft x 9ft. It has been under lights for 2 weeks now. After the first few days I potted the seedling into a 25 litre pot, but I moved it into a 17cm diameter pot today after it being in there for over a week thinking that I had potted on too early. I am now only feeding it with Volvic ph'd to 6.5 since I suspect chlorine toxicity. I also wondered if it might be nute burn & that the ProMix might be too much for it. I have only given it one dose of liquid feed altogether. Any advice would be helpful as all of my Widow seeds have developed like this

is this promix like miracle grow or what, and it has nutes in it..and your feeding more?

I only fed it once , because I thought it might have a nute deficiency. The leaves were already turning brown before I gave it the solution