Mg deficiency ?

This plant was grown from seed and i have no idea of the strain. Its an indoor soil plant under a 400W HPS light. Temperatures have been high these last couple of weeks between 90-100F. Latley it has been below 90F. I would like it lower but I cant control this at the moment. Anyway, I have just noticed the tips of all the leaves turning yellow and curling up. I dont think I have over fertilized it, but then again i do not have a lot of experience to tell..

Would this be a Mg deficiency ?

Thanks for your help guys

either first sign of nute burn or your using tap water if nute burn back off the nutes a tad, if your using tap water figure out a better way to remove the chlorine

I never thought about the tap water! The only other source of water i have is RO water which would be perfect.
Provided that its not nutrient burn, then this sign is a high chance it is the chlorine in the water and not any other problem ?