Yellow Tips On My Plant

[Fredfarmer]

I am new to growing. This is my first. Plant has been growing in soil for about 40 days. My plant grew to within 6 inches of my grow light. Its a 250w HPS. The grow area has a 12 in fan with a exhaust fan. I have since moved the plant to about 18 in from light. The plant started to yellow at he tips. My nutrients are FloroDuo A B Ca. The ppm are 300. Ph is 6.1. I feed in every other day about 1/2 gallon. The plant drains very well. I have Fox Farm
soil with perlite. My question is this a light problem or nutrient burn?

Ref pics.

 

[TheMadDabber]

Looks like heat stress to me. 6 inches is too close imo

 

[The Germinator]

Yup heat stress from the light being too close. Possibly nute burn too. Feeding every other day doesnt sound good. I think it's a combo of the two.