HELP, nutrient lockup or fertilizer burn?

[Smokin'N'Growin]

I have a few small plants growing in soil, indoors. The plants have been growing well, except about a week or so ago they started yellowing at the very tips, then slowly working down the edges of the leaves. This part would look burnt and dead. The weaker plants have stopped growing and are completely yellow and wilted looking, I imagine these will die. The stronger plants, however have continued to shoot another set of leaves, however these leaves have a yellow tint to them and curl upwards just a bit. The lower leaves continue to show the signs of burning, and is spreading slowly.
I have used Clearex, a salt leaching solution to strip the soil of nutrients just in case lock-up was the cause from overfertilizing. Some showed improvement, others continue to worsen. I don't know whats next, I can tell that whatever it is is slowing the vegetative stage, they're not growing as quickly as they should. I've go them under a 500W HPS, and it gets quite warm in there, however from experience I don't believe this is the cause. I will try to post some pics when I get a battery for my cam, but I need to do something quick, hopefully someone will have a correct diagnosis- these plants are destined for my lungs, i wont have it any other way!

Thanks guys-

 

[UK Hippy]

I cant comment on soil nutrients because I uses hydro but I get like golden tips on some of my plants but they nether died. Have a look at the cannas range of nutrients and see if they do soil nutrients