1st Grow Yellow Spots only on Top 1 wk into flwr

Hi All 1st time grower here stuck on a few things. My room is in the basement and is 3 x 4 7ft high. 600w light 1 fan in room. I have 2 plants to start and 1 has started to go yellow on a few leafs on the top with some dark brown on the bottom leaves. They are only 1 week into flowering and I've only given them nutriets once I mixed 50 ml A & B into 4l of dist. water. Then next day I raised the plants 1ft and a couple of days later I noticed the coloring of the leaves. So I lowered the plants and started just giving them dist. water only and nothing has really changed. I checked the PH today and it looks like its at 7 so I think thats a little high so I need to add vinagar. But the yellow could be nut. burn or light burn not really sure. Also I see tiny white specs that can be below off so no I'm concerned about bugs. I've attached some pictures so if any has some advice I really could use the help....Let me know if you need me to focus pictures on a certain area and I will try again....Thanks..

Don`t worry to much for your first grow id say your doing pretty amazing compared to most people, i think your main problem is your pot size, you should transplant those plants into double to size pots at least, this is mostly the reason for plants to start stressing and leafs at the bottom turning brown, forget about the nutes and the ph atm, its not your biggest concern, also, how high is your light from your plants? the light should be aprox 30-40cm above plants for a 600 W "HPS" is it?