Yellow leaves and burnt leaf tips

My very first grow and I'm getting some yellowing and now brown spots on some of the large leaves that don't get a lot of light. Also have some slightly burnt leaf tips which may be from less oscallating fan over the several days.

What type of medium; soil or hydro? - Coconut Coir
What brand and type of soil? - Canna
Indoors or outdoors? - Indoor
What strain? - Strawberry Cough
How old are the plants? - 10 weeks
What type of lights and how many watts? 600 Watt HPS
How far from the lights? 12 to 18 inches
What is your watering frequency and source of water? - Daily, tap
What, how much and when was it fed? NPK? - Canna A & B 1.5 ml per liter additionally I'm using Root Excelurator .5 ml per liter.
What is the medium/runoff pH and PPM if in hydro? - don't know. Ph of the water solution before application is about 6 to 6.5.
What are the temps and humidity in the room? 75 to 80 degrees F
What size pots? 5 gallon
Any bugs? Look real close. - Had fungus gnats on and off, treated with Neem oil 9ml per liter in the daily watering water
Any other pertinent info? - Started 12/12 light 6 days ago. Big leaves were already getting yellow and have been for some time, just more progressed now.

Images:

http://lockatron.com/public/checkup sept 24rd/

 

Ok no problem. I boosted my Canna A & B added more Root Excelurator (N and Potash) and I'm reading up on the Epsom salt thing. Wondering what's the best way to use it? I see on a few gardening sites they say 2T per gallon once a month in the ration water.

Does that sound right?

 

thank you sir. I'm on it.