Kinda looks like light bleaching from here, but hard to see with the pics under the hps.

Also, I would transplant soon into a 5 gallon container. 3 gals are too small for plants from seed. Also your ph going in is 6.5, coming out is 6, so the ph of the soil is less than 6. Could see some lockout soon.
bigtopsfinn Reviewed by bigtopsfinn on . top leaves yellow/includes troubleshooting form What is your experience level? (first timer, novice, experienced...) FIRST TIME Your Equipment: .1) Type and wattage of lights. (MH, HPS, CFL's, tube fluorescents, LED's) 600W hps .2) Distance from tops? 18-24 INCHES .3) Reflector type? (cool tube set-up, bat wing, enclosed reflector, bare bulb...)COOL TUBE .4) Is there a consistent fresh air supply? YES BUT NOT THE FRESHEST:( .5) Do you have an exhaust fan and a circulation fan?YES .6) What are the bulb wattages, kelvin ratings, and Rating: 5